To enter these codes you must Pause the game and enter them using the buttons on the N64 controller

Low Gravity - Pause, then enter Z R Z R Down R R Right Right R

Low Friction Environment - Pause, then enter Up R R Left Z Z Down Left Up Right

Heavy Rocket - Pause, then enter Up Right Right R Right R Z R R Up

Super Speed - Pause, then enter Z Right Down Up Down R Up Down Left Up