Hidden Tracks:

U.S.A.:

Use the boat with the big propeller. Go off-road to the left. Some of the hills are tough to get up, but at some parts of the track, the hills are very small. If you get up you'll find many awesome jumps, a few gorges, and a lot of ways to get flipped over.

Japan:

Use any vehicle. Go off road to the left again. Or if it's too much trouble, at the first "Gate
jump", follow the water to the left. You'll find a "Gate jump" surrounded in slippery water. To top it off, the track's surrounded by incredible jumps.

Car select:

Start the game with the -car -car command line parameter. The indicated cars will be available on the car selection screen. Valid car names are named as .VHC files in the game directory.