Enter these in the Pause menu:

Opens Bonus Gallery (can be used multiple times):
Triangle, Up, Circle, Right, Down, Down, Square, Left. 

Restore Life: 
L1, R2, L2, R1, Up, Up, Left, Square, Right, Circle, Start. 

Shrink Jamgut: 
R2, R2, Circle, R2, L2, L2, Left, L2, Triangle. 

Different Ending: 
Save all of the Natives.