Cheat codes screen
To access the Cheats Codes screen, at the options "Stamper" screen Hold L1 + L2 + R1 + R2 + Left + Circle + Triangle + Square and press X
Cowboy Hat Easter Egg
After killing BigBart in the first level his hat will be on the ground. Touch the hat to wear it. You will lose the hat if you get killed.
Sound Easter Eggs
Listen to the first six "intro" sounds in the sound test. Keep listening for a few seconds after the instructions and reports. Tiny will say random useless facts, "a special voice mail from a very special little girl" and recording of the game testers singing the "Tiny Boy" ending theme song..