Secret Cars
Win world championships with various cars to unlock the secret cars in the game.

Get Ambulance
Beat the legend Circut level with the pick up truck to get the ambulance.

Guns Mode
As soon as you see the main menu quickly press up, down, left, right, hold A, and press start. If you have done it fast enough, there should be a white box on the screen that says choose your weapon. Choose from a machine gun, flame thrower, etc. Then press start and start playing on any level you select ( yes, even multiplayer. )

All 12 Extra Cars
You can get 12 extra cars by doing the following:

Street Rocket - Beat the Novice Circuit
Taxi Cab - Beat the Amateur Circut.
Pickup - Beat the Professional Circut.
Ambulance - Beat the Legend Circut.
Baja Buggy - Beat the Time Trial for Destruction Junction with Street Rocket.
Rag Top - Beat the Time Trial for Alpine Ridge with Baja Buggy.
Blue Demon - Beat the Time Trial for Seascape Sprint with Ragtop.
Hatchback - Beat the Time Trial for Terminal Impact with Ragtop.
Low Rider - Beat the Time Trial for Metro Challenge with Ragtop.
Woody Wagon - Beat the Time Trial for Bayou Run with Ragtop.
Hot Rod - Beat the Time Trial for Sunset Canyon with Ragtop.
Police Car - Beat the Time Trial for Midnyte Rumble with Hot Rod.