Shrek

Perform at least four good deeds to unlock race mode. Play in race mode to get tokens to buy and unlock the following cheats: Chalk Mode, Crude Oil Mode, Cyber Shadows, Fast Mode, Infinite Flame, Infinite Gas, Invulnerability, Leak Mode, Low Gravity, Money For Merlin, Plastic Mode, Super Flatulence, and Super Strength.

Anywhere in the game that you can manage to fall off the main game map, you will be teleported to the chicken experimentation room. Note: You cannot get out without restarting the XBox. Some ways to get into the room include:

Sweetsville factory
Unlock the low gravity cheat and jump on the roof of the building that holds the power switch. Jump off into the water behind it.

Prince Charming's Castle
Jump out of the window that you are supposed to kick the prisoners out of.

Doing 5 good deeds will unlock the Creepy Crypt.

At the Title screen, press Up, Down, Down, B, A, A, Z, Down, Up, Left, B.

There is a secret area that is accesible from the room with the dungeon guard (where you kick the prisoners out the window). Do the wall jump until you fall out the big window, you'll fall for a very long time, and then end up in a room that apparently has no exit. The room is full of chickens and has green orbs suspended from the walls. While in this room even the START button won't let you exit the mission.