Grind on something and do a back flip or front flip (Down + R or Up + R). Land back on the pole and do it repeatedly.
Get All Places To Ride
To get all the places turn your gameboy on, when it gets to the main title where it says press start and type in the code L R L R B B R L and you will get all the places to ride at.
Unlock Metal and Flash
Go to the title screen and press R, R, L, L, B, B, L, R. A sound will indicate correct code entry .