Hold L, C Right, C Down, and the A button at the 
main menu, then enter one of the following codes: 

MOWER Invincibility 
XLURIDER Transparent rider 
BLAHBLAH Big Head mode 
SHOWOFF Stunt Bonus 
INVISRIDER Invisible riders 
IMGOINGNOW Debug mode 
MIDNIGHT Night mode 
YADAYADA Mirror mode 
TRICKSTER All stunts 
PINHEAD Small head mode 
ROTCOLS Weird colors 
PATWELLS Beat This! mode 
UGLYMUG Developer team photo in credits