Go to "The Cheat Menu" and enter one of the following:

The all tracks code will exit the code entry screen and make a sound to confirm the correct code entry. The other codes only make a sound to confirm the correct entry and do not exit the code entry screen.

MUNCHKIN - First Additional Greeting At End Of Credits
EDDROOLZ - Second Additional Greeting At End Of Credits
EDDIEPOO - Third Additional Greeting At End Of Credits
FREEPLAY - Unlock All Levels Of Racing
WIBBLE - Unlock All Tracks
UCANDO - Unlocks O2 Liveried