At the password screen in the options, you can enter the following codes:

CAR CHEATS R1 S X S L2 O=ALIEN SAUCER T L1 R1 X L2 L2=GOLIATH R1 R2 L1 L1 X O=NIGHT SHADE L1 T R2 T T R1=HELICOPTER LEVELS:(CHALLENGE MODE) X O L2 X S L1=FUNTOPIA X S O L1 L2 S=GULTCH

OTHER CHEATS L1 L2 O L1 R1 S=DOUBLE PICKUP O S R2 X T R2=INFINATE JUMP S X O T R1 R2=INFINATE TURBO R1 T R1 T L1 S=INCREASED ARMOR