All levels open and credits:

At the title screen, press Right, Up, Up, Down, Down, Right, Left, and Up. If done right, a sound should be heard. Now you can press Select to view the game's ending or you can start the game and press B on the map to go to any level you select.

 

Soft reset the game:

At any time during gameplay, on Controller Two, press Left + Select + Start to reset the game.