The Tunis light rail (Metro Leger) is an above-ground tramway that covers the city cheaply and reasonably well, though stops are not always clearly marked. The TGM suburban train runs from central Tunis north to La Marsa, passing through Carthage and Sidi Bou Said -- it is the easiest way to reach those destinations.
Taxis are plentiful and inexpensive but insist that the driver uses the meter. Driving yourself is possible but stressful -- traffic is chaotic and signage is poor. For the medina and Ville Nouvelle, walking is the best option.