Begin the game as Max, then hold R2 and press Up 4 times.
Lounging Kurt:
At the main menu hold L2 + R2 and press Square, Square, Triangle, Square.
Kurt In His Boxers:
At the main menu hold L1 and R1 and press Square, Square, X, Square. Remove menu from pause screen. Pause the game and hit L1 and R1.
A Hydra MDK Style:
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.
Panning Camera:
Pause game play, then hold L2 + R2 and press Circle, X, Circle, X.
Crazy Camera Views:
Pause the game, then hold L2 + R2 and press X, Circle, X, Circle.
Dr Flagulence...er Doctor Hawkins:
Begin the game as Doctor Hawkins, then press L2 + R2 + Left + X.
Invincibility for All Characters:
While at the pause screen enter while holding R2 and L2: Up, Up, Down, Down, Left, Left, Right, Right, SELECT (hold until pause screen disapears). *This code goes away after the level that you entered it on is over, so you must re-enter it for every level.