Hidden Cars:
To access the school bus, Jeep Wrangler, or police car - press the top, left and bottom C (yellow) buttons simultaneously at the Car Selection menu.
Hidden Levels:
At the Stage Select Screen, enter (do these by holding down all buttons at the same time):
Golden Gate Bridge - L + C + Left + C + Down
San Francisco - L + C + Down + C + Right
Indiana - L + C + Right + C + Up
Get the Fire Truck: - from C.J. Ehley
You first must get the school bus (see above) and race all the difficulties. On the last difficulty when you cruise the USA and win, you'll get the fire truck.
Faster Cars: - from C.J. Ehley
Usually when you start a 2-player race across the US, only 1 person gets the upgrade. But, when 1 person selects 1 save file and the 2nd person picks another, you can stop that. First pick different save files, then different cars. Finally, race up to Washington DC. Pause and quit the race. Then reset - do not turn off and on the machine. Re-enter the game using the files used in the the 2-player game. Finish the race with both cars and you'll have 2 upgrades.