Very close to Tunis, Carthage is a place you must go and see. There are numerous ruins spread out over. The highlight for me were the Antoine Baths. You can wander around and picture what was here before the Punic wars. Other areas well worth visiting are the Punic ports, the tophet which is a cemetry with interesting tombs, and the Carthage Museum. You really need a full day to explore fully and try and do it when it isn't too hot.