The Haunted Mansion

Zeke is a skeleton:
Hold A + B + X + Y at the opening sequence, before the legal screen. Release the buttons at the title screen.

Zeke is invisible:
Hold A + B + L + R at the opening sequence, before the legal screen. Release the buttons at the title screen.

Invincibility:
Hold Right, then press B, X(3), B, X, Y, A during game play.

Weapon upgrade:
Hold Right, then press B(2), Y(2), X(3), A during game play.

Level select:
Hold Right, then press X(2), B, Y(2), B, X, A during game play.

Full health:
When low on health, save then quit the game. Reload the saved game to resume with your health restored.