The Ice Key and Coloured Eggs
As seen in the the fourth and "best" ending and if you looked hard enough, you can collect the Ice Key and the Couloured (question mark) Eggs in this game. Originally the plan was to connect Banjo-Tooie with Banjo-Kazooie to get those items, but you can collect them by entering the puzzle-letter room in Treasure Trove Cove. Enter the following codes for the following prizes.

CHEATNOWYOUCANSEEANICEKEYWHICHYOUCANHAVEFORFREE - Inside Wozza's Cave in the Freezeezy Peak level, get up to the ledge where the Jinjo was and enter the room behind it for the Ice Key.

CHEATOUTOFTHESEAITRISESTOREVEALMORESECRETPRIZES - Inside Shark Island in the Treasure Trove Cove level, climb the tall pillar to the top for an coloured egg.

CHEATAMIDSTTHEHAUNTEDGLOOMASECRETINTHEBATHROOM - In the bathroom in the shower near Loggo the toilet, inside Mad Monster Mansion, there lies a coloured egg.

CHEATDONTYOUTELLHERABOUTTHESECRETINHERCELLAR - In the cellar of Mad Monster Mansion inside one of the barrels is another coloured egg.

CHEATTHISSECRETYOULLBEGRABBININTHECAPTAINSCABIN - Inside the captain's cabin in the Rusty Bucket Bay level is another couloured egg.

CHEATNOWBANJOWILLBEABLETOSEEITONNABNUTSTABLE - Visit Click Clock Woods in the winter and Beak Bust into the window of Nabnut's home to see the coloured egg on his table.

Cheato the Spellbook
On the Overworld Map on the first two pages, you might have noticed three little spellbook icons. They weren't talked about then because they are indeed secret areas.

The Spellbooks tell you codes that'll double your eggs (to 200), red feathers (to 100) and gold feathers (to 20). It seems you can only activate them after finding the spellbooks-no cheating! Here's where and how to find 'em (also see the Overworld Map):

One: Left of the puzzle picture for Freezeezy Peak is a pipe blocked by a boulder. Break the boulder and return as the alligator. You'll get the "BLUEEGGS" code here.

Two: In the same red-lava room where the puzzle picture for Gobi's Valley is, there's Brentilda. Behind this nice witch is a small hole. Turn into the pumpkin and enter it. You'll get the "REDFEATHERS" code here.

Three: After getting the Overworld puzzle piece from the switch in Rusty Bucket Bay, break the grate on the wall behind it. At the top is a water switch. Hit it and swim to the room with the entrance to Rusty Bucket Bay. Before time runs out, go to an alcove only accessible when the water's high. You'll get the "GOLDFEATHERS" code here.

Easter Egg Codes
Is any Rareware game complete without kooky codes? No! Fortunately, the game's developers made finding these codes fun, thanks to secret minigames.

-Where Do I Go?
Return to Banjo's home. Stand in front of the fireplace and look up at the picture of the mole using Up-C. It may not happen automatically at first, so give it a couple of tries. If successful, the mole will say you've discovered his hidden puzzle game. (A final note: It doesn't seem like this hidden game is available at the start - you may have to beat the game completely first.)

-What Do I Do?
There'll be a series of puzzles to put back together. Use A to pick up and drop pieces; use the C buttons to rotate pieces. The difficulty comes from the constantly moving picture. The best advice: Find the corners and edges; place those first. The puzzle pieces always seem to be placed in the same locations each time - making them easy to memorize. Press Start to give up all hope and restart the puzzle; however, any puzzle except the last can be solved even if mistakes are made early on.

-What Do I Get?
One code is unlocked every time a puzzle is solved. Return to the letter-stomping ground in Treasure Trove Cove. The codes don't work unless the puzzles are solved first - no cheating!
Puzzle 1: BOTTLESBONUSONE (Big head for Banjo)
Puzzle 2: BOTTLESBONUSTWO (Big hands and feet for Banjo)
Puzzle 3: BOTTLESBONUSTHREE (Big Kazooie)
Puzzle 4: BOTTLESBONUSFOUR (Tall and skinny Banjo)
Puzzle 5: BOTTLESBONUSFIVE (Tall, skinny Banjo with big hands and feet)
Puzzle 6: BIGBOTTLESBONUS (Big everything, including Kazooie)
Puzzle 7: WISHYWASHYBANJO (the washmachine Banjo)

-Is There More?
The mole may say he's done, but look at the puzzle again for one more tough puzzle. He'll give you the WISHYWASHYBANJO code. As the name implies, this code transforms you into a washing machine. This form doesn't seem to have any more abilities than the normal form, other than the ability to walk through some dangerous water areas without taking damage.

-What then?
To make these codes work, go to Treasure Trove Cove. Enter the puzzle-letter room and type out the codes by stomping on the letters. You cannot enter any of the codes shown unless you beat the puzzle game that gave you that actual code. To deactivate any codes, go to the puzzle-letter room and type in NOBONUS. The cheats you got from Cheato don't work with the NOBONUS code.

More Codes
Enter Treasure Trove Cove and enter the sandcastle. Enter one of the following codes on the floor. You'll hear a moo as you enter CHEAT, but you won't get any other indication that what you're doing is having an effect until you finish the whole code.

Infinite Blue Eggs - CHEATBANJOBEGSFORPLENTYOFEGGS (your egg total will will not change, but you'll a endless supply).
99 Mumbo Tokens - CHEATDONTBEADUMBOGOSEEMUMBO (your Mumbo Tokens total will increase to 99).
Infinite Red Feathers - CHEATNOWYOUCANFLYHIGHINTHESKY (you will now have an endless supply of red feathers).
Infinite Gold Feathers - CHEATAGOLDENGLOWTOPROTECTBANJO (you will now have an endless supply of gold feathers).
Infinite Air - CHEATGIVETHEBEARLOTSOAIR (Banjo will now have infinite air when swimming).
Infinite Lives - CHEATLOTSOFGOESWITHMANYBANJOS (you'll now have infinite lives).
Maximum Energy - CHEATANENERGYBARTOGETYOUFAR (this will give you eight honeycombs on your life bar. Note: if you already have red honeycombs, you'll lose them.).