My visit to Tunisia was on 2008 on a tour around the country, so not the most up to date info, but no one answered up to now, so I thought I would share my opinion for what it worths...
On Tunis there is a great museum called Bardo museum, that hosts a great sellection of Roman mosaics and other items such as a few statues and ceramics from an ancient ship wrecked possibly carrying Greek artwork to some emperor in Rome or something. According to some scholars mosaic collection of this museum is one of the best in world. Building hosting the museum is interesting too.
Tunis old town area with its mosques and market is an interesting place too. Many cruise visitors tend to feel stressed because they are not prepared for the Arab way of things though. Shopkeepers are very pressing, they'll always try to talk you into their shops/stalls, they may even chase after you, when you enter, the concept of "just having a look" is not understood by them, they are dedicated to sell you something and of courser bargaining is a must. Starting price will be rediculously high. Women are safe even solo but they need to have some major understanding of local culture. I met women complaining of getting husstle in Tunisia while they were dressed very unappropriately by local standards. Even on beach resorts, women should be "covered" away from the beach so as not to attract local men. By covered I do not mean wearing a bourka or anything, but it is wise to cover your shoulders, and legs at least below knee leght. Some loosy cotton T-shirt and trousers or similar is ideal on a hot climate and seem respectful on locals. Wearing hot shorts or tank tops is like asking for attention, even if you are on your husbands companion.
As with all places with crowds, there is some possibility of petty crime, pick pocketing also. Usual precaution must apply, and note that usually victims are men more than women, because men tend to be less street-wise to women and very self-confident that it won't happen to them, so they end up acting stupid like keeping whallets at back pockets etc.
Often visitors to North African countries worry for political situation, but I do not find this a concern for a visitor been on the usual tourist sites. If you happen to walk by a demostration or you feel some tention near a public building, it is easy to walk away and avoid any kind of unpleasant surprise. As far ad it is evident you are a tourist, no one is going to harm you.
Carthage is an interesting place. There is a wide area excavating that would take best part of a day to explore in depth. Most visitors concentrate on the Antonine baths. Large impressive ruins but I think some reading before hand or a guide is necessary to appreciate what you look at.
Frommer's is a bit slow right now with swifting to new format and everything, but you may want to check Tunisia trip advisor forum. Many members there are people focusing on beach resorts, but there are a few frequent members focusing mostly on antiquities or visiting Tunisia for decades, so they have a wider point of view and useful knowledge on the area.
Enjoy your trip!