When you start the los angeles level, there will be a wire to the left of the stairs. Grind till you get to the top of the building (you might need to keep jumping on the wire) when you get onto the top, boneless onto the motorway and skitch a car going right. Keep skitching until the motorway ends then jump off. You can now explore inside some of the buildings by the petrol station.
Original Tony Hawk's Pro Skater Tony Hawk
Enter "BIRDMAN" as a code. Choose the "Secret Characters" option at the character selection screen in classic or free skate mode to access the original Tony Hawk.
Perfect rail balance
Enter "TIGHTROPE" as a code to unlock the "Perfect Rail Balance" option at the cheat menu during a game.
Run
When you are not on the skateboard, hold down X to run.
Skip Berlin
On any difficulty, when you start the level, just go to phil any time and he will ask you if you want to go and just say, yes.
Unlimited Supply
When you find the Voltraman, press down Square and he will throw electric balls. It takes a while for them to disappear.