I recently had a very good experience with a car rental agency in Fira on Santorini. I found them on the internet - possibly on Kayak - and reserved on line. The manager, George, was very responsive in dealing with my reservation and the car was ready when we arrived.
We rented a nice little Kia Picanto for 38 euro per day and the mileage was very acceptable. We explored the whole island for 20 euro in gas. The agency is centrally located about 1/4 mile north of the Fira town center. I would highly recommend them.