To become invincible, go to the main title screen and press UP, DOWN, DOWN, B, A, A, X, DOWN, UP, LEFT, B. Now Shrek cannot take damage!

To unlock Race Mode, complete four good deeds. Race Mode will allow you to compete for tokens, which can be used to purchase cheats.

Once you unlock Race Mode, you will be able to compete for tokens by completing previously completed deeds in a certain amount of time. Each cheat can be purchased for three tokens. The cheats are as follows:

Invulnerability
Infinite Gas
Infinite Flame
Super Strength
Super Flatulence
Leak Mode
Fast Mode
Low Gravity
Money For Merlin
Chalk Mode
Crude Oil Mode
Plastic Mode
Cyber Shadows

Do five good deeds and you will unlock Creepy Crypt.