I'm surprised no one has mentioned using my personal favorite - air bags.
All jokes aside - With todays goofy legal system you really have to be careful using booby traps to protect a vehicle. After repeatedly having fuel stolen years ago I rigged up a mouse trap by tightening the spring and TIG welding a razor blade across the striker and set it behind the fuel door. End result was a ****** car, a 17 yr old kid with severely cut fingers and far more money spent in legal fees than I had lost in fuel. While you can get pretty creative protecting an occupied dwelling, the law takes a dim view on creating injuries protecting property.