Jeff Gordon Racing XS

At the "Car Select" screen, type "widebody" (without the quotes) and press [Enter].
This gives you the secret X-wave car.

Also at the "Car Select" screen, type "refury" (again, no quotes) and press[Enter].
This gives you Jeff Gordon's car.

Finally, at the track-selection screen, type "freedrive" and press [Enter].
This opens up all the tracks.
(Information from Cheat Machine)