At the screen where you pick your car, press C-UP, C-LEFT, and C-DOWN. Now you get to pick secret cars!
To get more tracks, go to the track select screen. For Golden Gate Park, hold L, C-LEFT, C-DOWN. For Indiana, hold L, C-RIGHT, C-UP. For San Francisco, hold L, C-RIGHT, C-DOWN.
When racing as the SCHOOL BUS or POLICE CAR, tap Brake, Brake, Gas to flash you lights...