Taxis are crazy cheap in Tunis, where they are all metered. From the downtown to the airport, it’s about 6-8 dirham, and from the Bardo to Carthage it’s 12-14 dirham. Sometimes they’re all full, and they’re hard to find at night but just be patient.
No one tells you that the Carthage archeological park is actually a combination of 6 sites, so keep your taxi if you can. Otherwise the walk in between each is at least 10-15 min, and the Tophet is pretty far.
If you can’t find a taxi back to Tunis, then just hop on the TGM, and take it back to Tunis Port, where there we are taxi’s aplenty.
Djerba is a lovely sleepy town in the south of the country. I really enjoyed wandering around the Houmt Souk, and finding these beautiful pieces of graffiti all over the place. I found a little cafe called Bacca and just sat there people watching for a while. The rugs are much reasonably priced here, since it was a small town. You can get a pretty sizable Berber rug for 150 dinars ($50).
If you’re into Star Wars, you can also go to the Matmata for a day trip, and go visit the troglodyte dwellings. You may also pass Ksar guiliane, which is a natural oasis, which you can swim in, but it’s kind of dingy.
Tozeur is another beautiful oasis, with a really lovely old Médina to wander through. I would recommend you stay at Hotel Ksar Rouge Tozeur.
From Tozeur you can go to Douz, the gateway to the Sahara, which mostly involves deserty things like camels and falcons. I don’t love riding camels, as it’s quite uncomfortable but at least they’re adorable to take photos with.