Press and hold L1 + L2 + R1 + R2 + Left + Circle + Triangle + Square and press X at the options Stamper menu.
Cowboy Hat
When you kill Big Bart in the first level his hat will be laying there waiting to be put on. If you get killed you will lose the hat.
Easter Eggs
If you listen to the first six "intro" sounds in the sound test, listen for a few seconds after the instructions and normal reports. You will hear Tiny give you random useless facts ("Poison Arrow frogs are the world's most poisonous animals"), a "special voice mail from a very special little girl," and a hilarious recording of the game testers singing Tiny's "Tiny Boy" ending theme song.
Invulnerability
At the Cheat Codes screen, enter TANKODOOM.
Level select
Press and hold L1 + L2 + R1 + R2 + Left + Circle + Select at the new game Stamper menu