Sometimes the brochures will give you a list of excursions available in advance, but if you are with a Tour Operator you will be invited to a 'welcome meeting' shortly after arrival, usually on the following morning. There you will meet your rep, who will be absolutely delighted to sign you up for as many trips as possible. Do be careful about signing up for things straight away, as it's the rep's job to get you to part with your money, and some of the trips may not be suitable for you. You may get a half-day trip to Sousse and PEK to give you an idea of the area and an introduction to the Medina. This is worth doing if it's your first time, particularly if it's free which it sometimes is. Be wary of any trips to see how carpets are made - you will invariably get the hard sell. You can sometimes watch ladies weaving carpets in the Soula Centre in Sousse and that is free.
Another interesting trip is to Tunis for the Bardo Museum, Carthage (interesting ruins) and Sidi Bou Said (absolutely beautiful blue and white painted houses covered in flowers but quite steep roads for walking) The 2-day Sahara trip is excellent though it can be very tiring for young children as you spend a lot of time on the coach. There is plenty of information on this on HT. The coliseum at El Djem is the third largest amphitheatre of the Roman Empire and the cave houses at Matmata were the site of some of the Star Wars films. These are often visited as part of the Sahara tour.
Frigua Park, near Hammamet has a good reputation and can be done either independently or via your rep.
There are 3 drag shows in Tunisia, of which Changing Ladies is the best, though the others aren't bad. They do a round of the hotels and Changing Ladies is also on at the Casino in Sousse on Wednesday nights I think. The Reps Variety Show is usually a good laugh.
Things to do on your own are the Aqua Park at Port El Kantaoui, which is a nice cheapish day (or half day) trip for the children. Boat trips, banana boats, parascending and such are better booked up direct with the boys on the beach (remember to 'negotiate' for a price :wink:) Also try and visit the Mausoleum of the late president at Monastir, which is absolutely beautiful and Monastir itself is worth a visit, with a different atmosphere to Sousse or PEK.
Bear in mind that transport is extremely cheap in Tunisia though you do need to 'negotiate' with taxi drivers.
You don't say how long you are going for but I hope this has given you an idea of some of what's available. You will find more details of most of these things on here, or you can simply 'google' them for more information. Hope you all have a lovely time. I'll be back in Sousse around that time and have put in an order for plenty of sunshine.