All options

While the game is starting, press A,B,A,L,SHIFT A,Z.

More color

At the players select screen, press Z,B,A,Z,A,L.

Play like Gargos

At the players select screen, press Z,A,R,Z,A,B.

Sky stage

To have the special sky stage as a vs. level when you pick your characters, both players hold down then press down C to choose your character.