Rental cars can be obtained at the airport or there are several rental agencies in Hammamet as well.  Camel Car is a good local rental option.

The best way to travel is by taxi. It's very cheap, fast, reliable and convenient.  You can find taxis everywhere.  Day rates tend to be less expensive than evenings.  Yellow cabs are a good option, and be sure to agree on a fare before you depart or make sure the meter is running - the word for this which the locals will understand for meter is 'counter'.