Here are some of the ideas I collected to run a Harry Potter-themed Scout Encampment or Day Camp.  Please email me if you have any ideas to add... Enjoy! 
Barb
bdhunter@aol.com
From Laura:
My son is attending a Muggles Magic class at a local shop.  He is 9 and was a bit disappointed (he had very high expectations I think) so far but they started out with a sorting hat where the kids drew a slip to see what house they are in.  Then they have a bit of a workbook.  There is positive feedback with points going to the houses for the House Cup -- although dealing with disappointment of being selected for Slytherin is a problem for some.
They have made a magic gloop (the material from the inside of diapers (I think you can get it at gardening shops), mixed with tempera paint and water) and made a broomstick with a pencil and straw.  And they made a snitch out of 'magic molding material' (don't know the full name) it is somewhat soft after drying in air.  They put two feathers in it and he added a jewel and they painted it gold.  He brought home "How to Play Quidditch (in the pool)", using noodles as broomsticks and a clear marble as the snitch (hard to see under the water).  We haven't had the right weather to try it yet. Let me know if you want me to retype it.
At home my Potter maniacs are playing in the capes I made (Halloween costumes well ahead of time) and making their own spellbooks by pulling spells from the books and writing them in black notebooks with milky pens -- while listening for the umpteenth time to HP books on tape from the library (after having read them all).
Laura  Ldr Cadette Troop 177
Monroe, NY
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GSCNC hosted a 2 week day camp with a Harry Potter theme.  Following are some notes from 3 different volunteer counselors at that camp.
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Barb's input:
Here in the Nation's Capital, my daughter attended a two-week day camp that was called "Hogwarts" -- everything was Harry Potter (if you've read the book, my notes will make sense)...  Girls were divided into different units.  No one was in Gryffindor or Slytherin -- it wouldn't have been fair, etc.  Instead, they were in units like Ministry of Magic, etc.  The Camp Director was (of  course) Dumbledore.  A lot of the activities were tied to Harry Potter -- they had a wildlife specialist bring in live owls to talk about them... a magician came by... Juniors in one unit earned the "Science Sleuth" badge and they did "Potions and Spells" -- making rock crystals grow, etc.  The units earned "House Points" for the "House Cup" (based on good deeds, clean units, etc.).  If you've read even just the first book (quick reading for an adult reader -- and a GREAT book), then you'll see wonderful and easy parallels.  They even had a Quiddich game (instructions below).  It's a wonderfully easy theme to work into an overnight or week-long camp -- you could do a "Herbology" class (plants) "incantations" (campfire songs) or all kinds of closely related items.  It was inspiring all the girls to read, too!  Good luck!
Cindy's input:
Yes, the Harry Potter theme was a fun one and could easily be adapted to an encampment.  Anyway...
Hi all-
In addition to transfigurations (Arts & Camp Crafts), the songs section was called "Incantations" and the Outdoor Skills component was called "Muggle Studies".  I thought that was pretty clever.  One of the song leaders wrote a song about Hogwarts especially for camp, which I can get to you if needed.  They also had a Games component in which they did play Quidditch (how they adapted it to play without flying brooms follows, below).  The Quidditch was probably a bit too hard/complicated for Brownies, but the Juniors managed it.  They also had a professional magician come and put on a show one day (it was great!).  A lady from the Virginia Wildlife Conservancy came with 3 live owls to explain about them and show them up close to the girls.
Each unit chose a House Name and made a banner and a cheer.  (Units were not named for the actual 4 houses in the Potter books to prevent hurt feelings).  House points were awarded throughout the two weeks for various examples of good behavior and also for the daily Harry Potter trivia.  This could be adapted to have each troop choose a house name and maybe the banner and cheer could be done ahead of time at a troop meeting.  The cheers were done each day at opening and closing flag ceremonies when unit roll was called.  You don't really need a prize for the winner - the girls are just thrilled to win!  Some of the house names were Ravenswing, Owlsclaw, Phoenixfire, Dragonfire, Fairydust, Ministry of Magic, Enchanted Unicorns, & Hedwigs.
During our unit time, we made magic wands (using clear aquarium tubing) and wizard hats, also decorated a small cardboard box (spray painted black) with silver & gold gel pens in stars, moons, squiggles, etc. and glued on a wooden ball (painted silver) on the lid.  The girls loved all the crafts!
We also made "mandrakes" which the girls loved!  This is done by placing 2 tbsp. of annual rye grass seed in the toe of a knee high stocking.  After putting the grass seed in, put in about 1 to 1 1/2 cups of potting soil, and tie up the end of the stocking.  Tap the stocking in your palm to make it into a rounded shape and then place it into a small plastic cup.  Glue on googly eyes with waterproof glue.  Then sprinkle the top of the stocking (where the grass seed is) with water and fill the cup with about 1-2" of water.  The "tail" of the stocking will act as a wick to bring the water up.  Keep watering daily, and in about 3 days the grass will start to grow - looking like hair!  (much like a chia pet!).  Since we were at camp for 2 weeks the mandrakes had time to really take off.  If you did it at encampment, they wouldn't sprout until after the girls got home, but I think they would still really love it.
There are many ideas for theme-related swaps...some units did something related to their house name; there were also wizard hats, magic wands, a small package of "Bernie Botts Every flavor jelly beans", (made from fimo clay, not real jelly beans), a goblet of fire (made with a bottle cap painted black) and small brooms.
Other activities we did were to play Human Chess (an easier version than regular chess, but still reminiscent of the way it's played in the books) and several other word search games and crossword puzzles that Nancy found on the internet.  We also had them create a maze and told them it could be the secret passages at Hogwarts, or the route from Hogwarts to Hogsmeade, or the secret passages under Gringotts Bank.  They had a lot of fun with that.
Our unit earned the Outdoor Creativity Badge and the Puzzlers badge during camp - these are easy to make fit with the theme.
I hope this helps a little... some of these ideas we got from recent issues of Family Fun Magazine.  They had an article on birthday parties which included a "Wizard" birthday party.  The wizard box idea came from American Girl Magazine.
Cindy
Here are the instructions you requested... I hope they make sense!
Quidditch:
Quidditch was played by having three hula hoops at each end of the field as the goals (the hoops were held up by some of the girls).  Each team had a "keeper" who stood in front of the three hoops and tried to prevent scoring (much like a goalie).  There were three "chasers" on each team who tried to score by using a broom to sweep a large rubber ball (the quaffle) down the field to the goals.  There were "bludgers" on the sidelines who threw nerf balls at the players of the opposing team, trying to hit them.  If they managed to hit a player, that player had to freeze and stay out of play for 30 seconds.  There were also "beaters" out on the field who tried to block the nerf balls from hitting their team mates, and also would throw the nerf balls back out to the bludgers on the sidelines so that they could keep using them.  Lastly each team had a "seeker" who tried to steal the "snitch" from the other team.  There were two snitches...each was a tennis ball painted gold-colored and was placed on the field behind the goal hoops (one on each end of the field).  The seeker had to try to run down the field to the opposing goal, grab the snitch and run back to their own end of the field without being bludgeoned with a nerf ball.  (I know it's pretty complicated, but so is the actual game of Quidditch on flying brooms as described in the book).
Human Chess:
We played a "shorthand" version of chess which worked well (got the idea from Family Fun magazine).  We used sidewalk chalk to make a grid on the concrete consisting of four rows of four squares each (total size 8'x8', with each square being 2'x2').  Then we selected 8 people to be the chess pieces - 4 on the black team and 4 for the white team - and 2 people to be the players  The chess pieces were given signs to hold with their name...each team had a bishop, rook, knight and pawn.  In this version of chess, no pieces get captured...the object is to move the pieces so that one player gets his 4 pieces lined up in a row, either horizontally, vertically or diagonally.  The various pieces move just as they do in the real chess game, with the exception that the pawn can move either forward or backward.  Everyone really enjoyed it and the game went fairly quickly.  It does take some strategic thinking, though.
Hope this helps!   Cindy
Jackie's input:
Cindy seems to have covered most everything.  Our Junior unit earned the Outdoor Creativity and Puzzler badges as well.  We also did invisible writing (I can't remember the specifics off the top of my head), simple magic tricks, and made a magic globe using a small jar, clay, a marble, water, glycerin, and glitter.  The Human Chess game was a bit hard to get going since it was a "short-cut" version and some of the girls had a difference of opinion about how each piece should move.  Having leaders that know chess would have helped!
My daughter's Brownie unit made edible fire using tootsie rolls and who knows what else but I'm sure I could find out.  It's something that she still talks about as one of her favorite things.  My other daughter's Junior unit made magic rocks and completed the Science Sleuth badge, which fit in VERY nicely to the magical theme of Harry…(they also finished the "now and then" badge and the ecology badge) (Note: badge and try-it work did not have to be a "Harry Potter" theme at all…)
My unit absolutely loved Quidditch!  I could help with the rules, too.  The mandrakes were a big hit as well.  My "Matilda Mandrake" is still going great guns and is faithfully watered daily by my Brownie who has adopted it as her own.
As for winning for the most number of house points, I'm not sure I agree with Cindy's thought that the girls were thrilled just to win.  I had one of the winners in my carpool and she was disappointed that only one girl received the prize of the Harry Potter book 4.  Originally, I had heard that they were going to give each of the winners small inexpensive plastic cups as a "House Cup" (ones that you would get at a craft store for weddings, etc).  Nothing fancy, but just a token.  I'm not sure what happened.
All in all, it was a fun week.
Jackie
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FROM http://www.oocities.org/EnchantedForest/Mountain/5101/party.html
Use Jelly Belly jelly beans for Every Flavored Beans!
Pin the horn on the Unicorn, Pin the scar on Harry, etc.
Print hand-outs from this website for your party!  This is okay provided you include the web site address   (http://www.harrypotterfans.net) on every handout.
Temporary scar tattoos!  You can buy these or "draw" them on!
Muggle Quidditch!  Click here for instructions!
Create Famous Wizard Cards with children's names on them... use as place-marks or party favors.
Decorate wizard hats or wands!
Make a magic elixir with various drinks... see who makes the best tasting!
Sort guests into houses for games (note: you may want to consider not sorting kids into Slytherin - kids may get upset or not participate)
Harry Potter Links
http://www.hootbooks.com/games/image/potter.jpg
Harry Potter word search (with nice illustrations)
Nice Crossword puzzle
http://www.oocities.org/EnchantedForest/Mountain/5101/crossword.html
GREAT bookmarks to color and make:
http://www.oocities.org/EnchantedForest/Mountain/5101/bookmarks.html
Great Harry Potter Craft ideas at http://www.makingfriends.com/readers_harrypotter.htm
New!!
http://www.oocities.org/harrypotterfans  --- where I got a lot of this stuff -- the Unofficial Harry Potter Fan Club page…
The Sorcerers Stone Cyber Lesson Guide:  http://www.connectingstudents.com/lessonplans/potter/
Scholastic Books Harry Potter Official Website:  http://www.scholastic.com/harrypotter/home.asp
New!! More teacher and craft links: http://www.dltk-kids.com/harrypotter/
Link for teacher ideas that has GREAT tie-ins for badges, try-its, etc...: http://www.oocities.org/EnchantedForest/Mountain/5101/teachers.html
The "Unofficial" Harry Potter Website (has lots of place names, character descriptions and some puzzles, etc.:
http://www.homestead.com/harrypotterbymegz/home.html
Harry Potter Sorcerer's Corner at Family Education website (has ideas for invisible ink, costumes, homemade trading cards, etc.) : http://familyeducation.com/topic/front/0,1156,22-16701,00.html
Connecting Students Sorcerer's Stone Site has some nice worksheets and puzzles:
http://www.connectingstudents.com/worksheets/books/hp&ss.htm
Lesson plans geared for teachers, but adaptable for scout leaders can be found at
http://www.studyweb.com
Harry Happenings Website (has book quote and word of the day feature):  http://www.kidsreads.com/harrypotter/happenings.html
There are some anti-Harry sentiments out there... heres a website with links to the pros and cons of Harry Potter: http://www.mugglesforharrypotter.org
Thanks Pat, Cindy, Nancy, Jackie and Laura!