I find that a written contract helps to clarify what the terms are, especially if something goes wrong.

-deposit or no-show charge?
-damage liability?
-what happens in the event of a mechanical DNF?
-what is included in the rental - full arrive and drive, or "here's the key"
-who pays for fuel
etc,etc,etc
-you may want to include a waiver to protect yourself in the event that your renter becomes injured while in your car

I once rented a car that came with a contract that included an agreed price for a driver caused engine blow up, and an agreed value for the car if it was totalled. I liked knowing that information in advance, instead of a battle after the fact.

Richard