Invincibility:
Press Start to pause the game, then press Down, O, Square, Square, Up, X. When you press Start to resume your game, your character will be invincible!

Level Select:
Pause the game, and enter the code: TOOLOUD (Triangle, O, O, Left, O, Up, Down) Then press and hold Select and Start until the game resets to the title screen. Go to the options menu and the first item is Test Start, which lets you explore any level.

Super Sword:
Pause the game, and enter the code: OLDTROUT (O, Left, Down, Triangle, Right, O, Up, Triangle).

Infinite Heartstones:
Pause the game, and enter the code: LUXSUDS (Left, Up, X, Square, Up, Down, Square).