The follow codes are entered at the Main menu, if you enter it properly, the screen shoould flicker....

All leagues, tracks, and snowmobiles 
hold R and press Up, Triangle, Up, Triangle, Up, Triangle 

ATV
Hold R1 and press Up, Right, Down, Up, Right, Down

Cartoon track 
hold R and press Right, Up, Left, Circle, Triangle, Square 

Demo mode 
hold R1 and press Up(3), Down(3) 

Go-Cart 
Hold R1 and press Right(2), Left(2), Right(2) 

Summer track
old R and press Triangle, X, Circle(2), X, Triangle