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