Get the required ammount of points in the Championship mode and recieve the following secrets activated on the car select screen.

8 Points: Power Level 1 (up or down c)
20 Points: Paint Job (L or R to select colors)
100 Points: Power Level 3 (up or down c)
150 Points: Two Tone Paint Job (L or R to select colors)
500 points: Power Level 4
1500 points: Power Level 5
9999 points: Speed Demon (Top speed 230)