Check List
Discs and Bag - OK this is real simple and real basic but I have seen people show up at a tournament and realize they don't have ANY discs with them. so before you leave the house and drive 3 hours make sure ALL of your favorite discs are in the car (that is the car that YOU are going to the tournament in!)
PDGA Card - This can save you money on registering at many tournaments and all sanctioned events. So if you are not a member...join.
Towel - Morning rounds tend to be damp with dew, and in the Baltimore area the saying is "If you don't like the weather...wait a minute it'll change". Besides most courses seem to have at least one small hidden supply of dampness, or doggie doo.
Extra Discs - These are real important if you only carry about 10 discs or so like I do and have never played the course you are going to. They are also nice when you break one of the ones you normally play with. I can tell you from personal experience that even your putter can break. If you don't wanna carry them that is fine just leave 'em in the car.
Extra Keys - Keep these in your golf bag. I have seen way to many people lock their keys in the car. I have even seen people leave their car running (by mistake) while they played and then realized their keys were locked in their car. You may forget your keys but you will probably remember your golf bag!
Water Bottle - So that you don't get dehydrated. This should be water or Gatorade or some thing like that, not beer. Beer is great for after golf festivities but it will only 'cause a faster rate of dehydration during play and will not help your game at all.
First Aid kit - Include band aids, antiseptic, aspirin, sunscreen, etc. Keep it in your bag
Diaper Wipes - These are great if you get into poison ivy or something. You can wipe the oil off before you get a rash.
Rulebook - Just in case you get called for an infraction you can prove to everyone that, yes, you did indeed falling putt.
On to the things that are just nice to have with you.
Money - Useful for registering, also nice for buying plastic or lunch or dinner.
Extra Towels - Handy if it is raining, 'cause one ain't enough.
Change of clothes - Nice if you took that between rounds swim to retrieve your favorite driver and were too shy to do it in the buff!!! Besides after a long day of golfing it feels real good to put on clean, dry socks and shoes.
Umbrella - In case it rains. This gets promoted to the bag in the event of rain-duh
Extra Water (or Gatorade or whatnot) - Keep that water bottle full so it can do it's job.
Directions to the course - Very important for courses that you have never been to before. The PDGA course directory can fill this roll very well.
TD's phone number - Unless he has a cellular phone you probably can't get a hold of him at the course, but at least you can leave a message.
Lunch and Dinner - Keeps you from getting cranky and running out of energy, avoid greasy foods like chips and other junk food. Now I know from personal experience that disc golfers are very friendly people and most will not let you starve, but be cool bring your own food!
Sense of Humor - Because all of us (except maybe Ken Climo, and The Mose) have bad rounds, and we don't need anyone biting anyone else's ear off!!
Friends and Family - Bring your spouse and Kids. Bring Mom and Dad. Galleries are cool!!
Your Game - Bring your game and be ready to show it off!!!