Debug mode:
Enter [Joker], [Joker], [Joker] as a password. Complete the game, hold Left + Select, and press I.

Game select:
Hold Select at the opponent profile screen and press Right (Blackjack), Left (Babanuki), Up (Poker), or Down (Speed).

Hidden picture:
Enter [Joker], [Joker], [Ace Of Spades] as a password.

Nude opponents:
Set the turbo switch on the controller to the "high" setting. Set the difficulty level to hard, then hold I + II when clothes are being stripped away after the final round.