It may be that someone you know is suffering at the hands of a disease and illness, and a car smash enables you to get your aggressions out as well as raising money to fund treatment or research at the same time.
Make some contacts with the junk yards, automobile dealers and auto insurance companies in your area to see if you can get hold of a car in relatively good condition (with just the body and glass intact). Before you get started you should make absolutely sure that all of the fluids and other hazardous materials have been completely removed from the vehicle.
You can then give participants a swing with a hammer at the car for $2 a go. Make sure that the proper safety precautions are taken: safety glasses and protective outerwear should be worn, and any spectators should be standing at a safe distance. Have paramedics waiting nearby just in case you need them. You might also want to contact your local authority to get permission for the smash as well as further safety tips.
Your local rescue squad might be willing to give a Jaws of Life demonstration if you explain the fundraising nature of the event. Advertising this aspect should bring in a larger crowd and more funds as a result.