Double Dragon 2: The Revenge

Level select:
Press I, II, I, II, Run at the title screen.

Level skip:
Pause game play, then hold I + II + Select. Resume game play while holding those buttons.

View animation sequences:
Hold I + II + Select at the title screen. Then, press Run as the screen fades. Press Up or Down to select an animation and press II to view.

Sprite test:
Press II, I, II, I, Start at the title screen.