Cruise ships sailing the western Mediterranan and docked at La Goulette, Tunisia often include a tour of Tunis as a shore excursion option. 
These tours include a walk around the Medina and one of the enclosed souks (shopping areas).
The city tours also will travel to the Bardo National Museum, which has the world's largest collection of Roman mosaics. Tourists might also choose to visit Sidi Bou Said, a small town near La Goulette and the remains of Carthage.
Tunis Souk (c) Linda Garrison
Tunisia is a tiny and charming country
I am happy that visitors share information about it. I would add, means of transport are various and cheap, you can easily discover Tunis and its suburbs, going from Bardo (where the musueum) to Carthage, crossing the Medina and the downtown. Please welcome!