Cheat Mode
At the "Press Start" screen, press L + R, C-Left, C-Right, L + R.
Press Start, then hold Z and repeatedly tap L at the mode selection
screen. The phrase "Cheat Mode" will show.
Bonus Cars
At the "Press Start" screen, press L+R, C-Left, C-Right, L + R. Go to the Options screen, then hold Z and press L.