Get all bonuses
Go to the cheats menu and enter: CLAPPEDOUT.
Open Up Pro Physics
Enter SWAPPIN in the character completion field.
50cc Class
Enter SQUIRRELDOG in the character completion field.
Unlock Freestyle Tracks
Enter L1, L1, X, X, X, X, Square, R2, R2, L1.
All Machines
Go to the cheat menu at options and enter at all machines 'MINIGAMES'.
Unlock supercross tracks
To unlock all supercross levels go to the cheat codes menu and put in the word "STUPERCROSS" (dont put the quotation marks in though.)
Unlock 500cc bikes
First go to the cheatcodes section and put in the code "BIGDOGS" (without the quotation marks) and you will get big bikes.