Enter the following at the "Press Start" screen. You'll hear an engine noise to comfirm each time you enter a code correctly.

Code	Effect
Left, Left, Right, Right, B, B, A	Unlock Radiator Cap Secret Circuit.
Left,Down,Right,A	unlock all screenshots at the Drive-In.
Right,Down,Right,B	Unlock all cars.
Up,Up,Down,Down,Left,Right,Left,Right,B,A	90 Bolts and all level unlocked.
Up,Up,Left,Right,Right,Left,Down,Down	Unlock all car colors.