All art gallery:
Select the Tweety icon at the start game screen, then enter .RT as a code.

All mini-games:
Select the Daffy Duck icon at the start game screen, then enter ?BN as a code to unlock all the mini-games.

Level select:
Select the Marvin Martian icon at the start game screen, then enter #OP as a code.

No whack in boxes:
Select the Taz icon at the start game screen, then enter !WB as a code.

Two player Boss games:
Select the Big Red icon at the start game screen, then enter *JC as a code.