I just spent 7 days in Tunisia in 2004. My itinerary is as follows:
D1: Tunis --> Sousse by train
D2: Sousse -- Visited Sunday market. By bus to Kairouan and visited Great Mosque, 3-door mosque, Barouta, Zaouia of Sid, Abid el-Ghariani, Maison du Gouverneur, Aghlabid basins, Zaouia of Sidi Sahab, Zaouia of Sidi Amor Abbada. Returned to Sousse by train.
D3: Sousse --> El Jem by bus. Visited Colosseum. Took louage to Mahdia. Visited Fatimid Fortification, Jewish cementary. Returned to Sousse by loage. Went to Ribat, Zaouia Zakkak, Museum Koubba.
D4: Sousse --> Mohadir by louage. Visited Ribat and Mausolem. Took bus to Sfax. Visited medina there. The medina I like most coz it is a vibrant rather than touristic community. Returned to Sousse.
D5: Sousse --> Tunis by louage. Visited Bardo Museum. Took train to Sidi Bou Said, a high class residential area. The houses here look like those in Greece with many colorful and well decorated doors! yes doors, the main attraction here. Returned to Tunis by train.
D6: Took train from Tunis to Carthage. Spent a lot of time in this UNESCO site. Returned to Tunis by train. Paid a visit to the medina of Tunis, another UNESCO site.
D7: Left Tunisia for Morocco by flight.