Cheat mode: At the Main menu select 'Options' then keep pressing L1 or R1 until you are transfered to the 'Cheats' option. Then press Circle which will display a keyboard that is used to enter codes to unlock cheats.
50cc and 500cc Classes: At the 'Cheats' screen enter 'SQUIRRELDOG' as a code for the '50 cc Class' option.
Pro Physics: At the 'Cheats' screen enter 'SWAPPIN' as a code for the 'Pro Physics' option.
Character Completion: At the 'Cheats' screen enter 'CLAPPEDOUT' as a code for the 'Character Completion' option. All bonuses in the game will now be unlocked.
Go to the Options menu and press L1 to go to the cheats screen, go down to where it says "Career Completion" enter the following: CLAPPEDOUT from now on, all the extras in the game, such as extra vehicles and tracks will now be unlocked.