Type "GO BRUINS!" at the begin career name entry screen for six new indy racing cars.

Type "GO BEARS!" at the Begin Career name entry screen for seven new stock cars.

During gameplay, press Pause. Highlight Race Statistics, then press and hold R + C or R + L + A to bring up the cheat menu.

Code - Result:

wyred - Wire-frame mode
phatcars - Very fat cars
pressstart - A new opponent