Enter any of the following codes when the words "Press Start" appear on the title screen; you'll hear a sound to confirm if you've entered a code correctly:
Hidden Cars - Up, Left, Right, X, Circle, Square.
Bonus Track - X, Up, Triangle, Down, R1, L1.
Reverse Tracks - Left, Triangle, R1, Circle, L1, Down.
Super Championship Race - Right, Square, Left, Circle, Up, X.