Maximum money:
Press Up, Triangle, Down, X, Right, Square, Left, Circle, Select, Start on the controller two at the title screen.

All parts:
Press Right(2), Down, Up(2), Left, Down, Left, Circle, Triangle on controller two at the title screen.

Secret parts:
Press Up, Down(2), Up, Left, Right, Left, Right on controller two at the title screen.

Alternate car display:
Press L1 or R1 at the car selection screen to display the vehicle's dimensions.