Here are some cheats for this game:
L, R, A (x5), B (x7), start and Select at the same time and at the title menu
L, R, B and A (x3), Up(x2), down (x2), start and select at the same time and at the title menu
Hold L and R, and then press A, Up, Down, left, Right (x4), B, Up and Start + Select at the title screen.
* For airheads:
Everyone's body will have their heads in the air while their bodies are on the ground just standing or driving.
Unlock All Cars
To unlock all cars, enter "ChUmP" as your driver's name at the New Game screen and all cars will unlock.