To enter all of these cheats, during gameplay press ESC, and click on options. Hold down [F1] while using the arrow keys to type in the following:

One hit kills the car: Right Right Left Left
Speedometer: Right Right Down Left
Unlimited speed: Left Left Left Left
Invincibility: Right Up Right Up
Fast acceleration: Right Right Right Right
Special cars turn to Lego: Down Down Right Left
Jump in car: Left Left Left Right (press horn)

Some other good things:
Set the date to 25/12/04 for Xmas mode
Set the date to 25/11/04 for Thanksgiving mode
Set the date to 31/10/04 for Halloween mode
Set the date to 04/07/04 for Independance (sometimes doesn't work)

Hope you enjoy them.