Invincibility:
Under "Settings"->"Cheats", enter IRONMAN as a code

Weaker police cars:
Under "Settings"->"Cheats", enter KEYSTONE as a code.

Completion Bonuses:
Beat the game to unlock the Atlus Racer, Ram Raider, and Negotiator Vehicles, as well as the "Then and Now" Era Change option.

Body Snatcher!!:
Get your odometer to 666, then melee attack a pedestrian to take control of them! Spooky!