Enter the following cheats the code screen under the options menu. 

Code - Effect 
LOTACRAP - Master code
MAXSTATS - Max stats for all 
PIKARIDE - All bikes for all 
DMCDMAN - Dennis McCoys stats are set high 
BONEGUY - Bob Jones stats are set high 
FLYAWAY - Bird Brains unlocked 
BADGIRL - Hot Babe unlocked 
SIGMAN - Angus Sigman unlocked 
OLDLADY - Ramp Granny unlocked 
OILSPILL - Unlock refinery 
CHOOCHOO - Unlock train station 
ARTRIDER - Unlock museum level 
ARTCOMP - Play Museum competition 
FUZYDIRT - Unlock Fuzzy's yard 
VOLCANO - Unlock Mt Magma level 
PAVEMENT - Unlock GG Street stage 
GGFLYER - Unlock GG Vertical stage 
MUDPUDLE - Unlock GG Dirt stage 
ANDFMV - Watch Andre Ellison FMV 
DMCFMV - Watch Dennis McCoy FMV 
JAMFMV - Watch Jamie Bestwick FMV 
LEIFMV - Watch Leigh Ramsdell FMV 
MATFMV - Watch Matt Berringer FMV 
REUFMV - Watch Reuel Erikson FMV 
FUZFMV - Watch Fuzzy Hall FMV