Just go to the goodies menu and simultaneously press L1, L2, R1, R2 and 
Select and a small OOOf will appear at the bottom-right of the screen now 
you know you can enter any of these codes.

Key:
4=L1 8=L2 1=R1 2=R2 3=R1+R2 5=L1+R1 6=L1+R2 e=L1+L2+R2

Codes:
Extra Characters=4e43
Additional Tracks=4154
Motortoon Tank Combat=5443
Submarine X=5358
Motortoon GP R=4631

Highlight option menu on main screen and hold L1,L2, R1,R2, and select, 
then press X for secret options.