In multi-player-mode, turn on all vehicles and go to a place with open space. Get in a Ghost and find a tank. Go towards it as fast as you can. You will flip and do different things, depending on where you hit the tank.

Choose a level with the yellowish glowing boxes that blow up (Headlong or Waterworks recommended). Get in a ghost and boost into one of the tiny boxes and you will be sent flying. You can repeat this as many times as desired, or you can pile the boxes and be blown sky high.

Go to any level (Metropolis recommended, when you have to get on the Scarab). Back the Ghost up to the side of the bridge. It will start hopping up and down.