1. You must try smoking the shisha (even if you are not a smoker - and I am not!!).
2. Try the mint tea.
3. See the Sousse Medina and visit the Soula shopping center (with fixed prices in a proper Western-style shopping center) in the Medina.
Quick Tips:
1. Best places to smoke the shisha are the two cafeterias on the Corniche seafront. One is called Andalus and the other is 50m away from Andalus on your right as you face the sea. Both are on the right of the El Hana Hotel complex (when you have the hotel behind you, they are 70 or so meters on the right).
2. The Soula shopping center has several items of quality and interest at reasonable prices, from home decorations and clothing to souvenirs. It is no-hassle shopping.
Best Way To Get Around:
The main touristy area of Sousse is about 1 square kilometer. It is a very easy, relatively flat walk. The Medina gives you a proper flavour of the Arab town market, with all the usual smells, noises, and flavours