The Medina of Tunis is perhaps the best Medina in the country, whilst the sites are nothing spectacular, and you can walk straight past them if you do not keep an eye out!, but the atmosphere of the labyrinthine streets is great!
The main way through is just an endless stream of shop keepers trying to sell you souvenirs, but if you duck off down some side alleys and get to some other markets you will start to see more of the real Tunis. It takes some time to really get acclimatised to Medina life but once you do it is great!
The central mosque I found to be a little disappointing especially as, not being a Muslim I had to pay just to walk through the gate to a very small viewing area to see a courtyard that was being renovated.
The best thing to do is just to get in and wander aimlessly, buy things from the vendors; especially the Barbary Figs (Prickly Pears!) they are sold on carts the whole way around the medina and they cost 100mils. (About ,08 € cents) and they even get peeled for you, go into a tea room and just sit with the locals who are ridiculously friendly, then go off and barter for some souvenirs.
Tip: if you want some roof top views many of the shops have terraces which you can go up to but don’t expect to get away to quickly when you come down to quickly as you will at least have to look at the carpets and pipes (at special price for you because they like you) or alternatively use a camera in front of the main mosque and within a few moments you will have some one showing you around with a few stops at their ‘brothers’ perfume shops, It is all part of the fun though!