Bonus Level:
At the title screen, press Select 12 times.

Invincibility:
Pause the game, then hold L2 or R2 & press Right, Right, Left, Triangle, X, Up, Circle, Left.

Debug Mode:
Pause the game, then hold L2 or R2 & press Left, Up, Up, Triangle, Right, Square, Left, Triangle, Up, Down, Right, Right.

Infinite Spirit Spells:
When you have acquired the Spirit spell, pause the game, then hold L2 or R2 & press Left, Triangle, Left, Left, Circle, Left, Triangle, Right, Circle, Up, Up, Down. You must repeat this whenever you pick up a Spirit spell.