Geometry Wars Mini Game:
Enter the garage and press Y to enter walk mode. Approach the area to the right of the garage door and you will find an arcade game next to a few tool boxes and some rims. Press A to play. This is the Geometry mini-game. To quit the game, press B, then A.

Wild Spinning Car:
You must have the Porsche 959 (extreme class) for this cheat. In the options, set Arcade racing to Manual shifting. Then go into any race with the Porsche. Drive until the car is at it's top speed in 1st gear (about 42mph). Then let off the acclerator, make a hard turn left, and change down a gear (usually X). Then press back on the power hard, and the car should spin wildly.

Thanks to Revolution readers Racer X and Richard!