Kurt in boxer shorts
At the main menu, hold L2 + R2 and press Square(2), Triangle, Square.

Mixed character
Pause the game, then hold L2 + R2 and press Up, Down, Right, Left, X, Triangle, Circle, Square, Square + Start to play as a character that is part Kurt, Max, and Doctor Hawkins.

Crazy camera
Pause the game, then hold L2 + R2 and press X, Circle, X, Circle.

Panning camera
Pause the game, then hold L2 + R2 and press Circle, X, Circle, X.

Slow-motion mode
While playing as Max, hold R2 and press Up(4).

Fart sounds from Doctor Hawkins

While playing as Doctor Hawkins, press L2 + R2 + Left + X.