Enter the following cheats in the Character Completion field, at the Cheats screen: 

Password  -  Effect:
BIGDOGS - 500cc bikes. 
BUSTBIG - Freestyle tracks.
CLAPPEDOUT - All bonuses unlocked. 
ECONATION - National tracks. 
MINIGAMES - Unlock all machines. 
OBTGOFAST - Expert A.I. 
SQUIRRELDOG - 50cc Bikes. 
STUPERCROSS - Supercross tracks. 
SWAPPIN - Pro Physics. 
WRECKINGBALL - A.I. Bowling.