I would suggest the Bardo Museum and maybe a short walk along the medina of Tunis.
Depends a bit of where you are going afterwards offcourse.
The airport is very close to the centre, only a few miles. A metered taxi should cost no more than around 5 dinars, depending on arrival time. I have never researched the bus, perhaps someone may have the details. Like taxi drivers everywhere they will try to charge more - if you agree a fare make sure it is dinars. Last time the driver tried to tell me he meant euros.
-:- Message from TripAdvisor staff -:-
This topic has been closed to new posts due to inactivity. We hope you'll join the conversation by posting to an open topic or starting a new one.
To review the TripAdvisor Forums Posting Guidelines, please follow this link: http://www.tripadvisor.com/pages/forums_posting_guidelines.html
We remove posts that do not follow our posting guidelines, and we reserve the right to remove any post for any reason.