Enter the following at the Codes screen:

TRAKMEET - Unlock all tracks
WTCHKPRS - Slow-mo
FTAIL - Low gravity
WHEELS - Unlock all bikes
LOKSMITH - Master code
POPULATE - Unlock all characters
YARDSALE - Unlock all costumes
FLYSOLO - No bike
FIRESALE - Constant flames
ALLFREEK - Freekout always full