During gameplay pause the game and enter these the following codes to unlock the corresponding effect.
Unlock the Big Ape Arena:
Circle, R2, R1, Circle, R2, R1.
Unlock Flat Land:
Circle, L1, L1, L1, Left, Right.
Unlock Infinite Energy:
Circle, R1, R1, R1, Down, Up.
Unlock Ape Mode:
Circle, L1, Circle, R1, Circle, L2, Circle, R2. Unlock Infinite Health:
At the 'Press Start' screen or when gameplay has been paused, press Circle, R1, R1, R1, Right, Left. If the code has been entered correctly you will hear a sound as confirmation.