Mini Kart: At the beginning of the Mario Kart GP mode, on the Character Select screen, press and and hold Y then press A. Your character will shrink to about half size.
150cc Class: Win the gold in the Mushroom, Flower, and Star Cup races in the 100cc class, then win the gold in the Special Cup race. If you do so, a new 150cc Class option will appear on the Class Select screen.
Race The Ghost Computer: When you start the game, enter the 1-Player Time Trial race, but when you are at the Player Select Screen, press CNTRL II START. A flashing COM will appear in one of the boxes above the drivers. Use CNTRL II to choose the computer's player, then choose your player and begin.
Switch Screens: When you begin playing a 1-Player Grand Prix, press CNTRL II: B. Using CNTRL II, select a 1-Player game, hold CNTRL II:[L]+[R] and press CNTRL II: START. Select the class/driver/track with CNTRL II and begin.
Extra Credits: You can earn extra credits by finishing 3 races in the same position.
Drive in Darkness: Play the Battle Mode. When a player is down to his last ballon, have him hold down B until he dies. Once dead, he will be able to drive around in the dark.