Level Select:
At main menu press L1, L1, L2, R1, R1, R2, Select

Unlimited Lives and Items:
Enter level select code, then on main menu press L1, R2, R1, L2, Select

Cut Everything in Half:
Enter level select code, then on main menu press L1, R1, L2, R2, Select

Play as a monster:
Enter level select code, then on main menu press L2, R1, L1, R2, L1, R1, Select. When you play as a monster you will become a different monster each level!

Baby Monsters:
Enter level select code, then on main menu press L2, L2, R1, R1, L1, R2, Select. You should see the word 'Reduce' - turn it on and check out the little creatures!

Mega-Password:
Enter as Password: Left, Up, Triangle, Down, Circle, Triangle, Square, Down. When you start, you can choose 'Infinite' Mode, Play as Monster, and Start Any Level.