Hi there,
I lived in the north of
Tunisia for a year. It is a fabulous country with genuinely hospitable people. Definitely take time to get out of the normal tourist zone -- Sousse, Monastir, Tunis, Carthage, Douz, Matmatta. These are very touristy places. Fine, if that's what you want, but you can be surprised at the richness elsewhere!
Travelling by train is very affordable and probably the most comfortable way to get from A to B. You can also choose a luage (8 passenger van) but you often have to wait for quite a while for these to fill up (they only leave the station once all 8 seats are filled).
I loved the Roman ruins throughout the country. By far the best ones I saw were in Bulla Regia -- up in the north-west corner of the country (about a 20 minute drive from the Algerian border). Another amazing sight, though more touristy, was Dougga. If you are looking for ruins you don't want to miss El Jem -- it has the largest Roman Colosseum ever built except for the one in Rome.
You gotta try a true Tunisian meal -- couscous, and salata mishwya. Delicious! In Tunis a great place for real Tunisian food (though a bit more pricey) is le Capitol. It is close to the port du
France on Habbib Bourgiba. Taxi drivers in the city will know it!
Have a great trip and Rubbi My-ik ("The Lord be with you" in the local Arabic dialect!)