Complete all the levels including the top set. Move up once more in the level select screen to access a row that does not display on the screen.
Goofy symbols
Call up the password entry window (with the four symbols) and rapidly press L1, L2, R1, and R2 until the symbols turn into slightly demented versions of themselves.
Go to the vehicle selection screen. Press and hold all four buttons (L1, L2, R1, R2). A vehicle will explode in the row that is highlighted. This can be done for each vehicle to make the game more difficult.
Suicide
Hold Square + X + Circle + Triangle to destroy the vehicle currently being controlled.