Ronan,
climb Yuelu mountain, eat some stinky tofu, walk along the riverbank (both sides), catch some street opera, dine in one of the cheap restaurants, hire a boat in Martyr park, check out the vibrant markets, visit the Changsha Love Bridge agency (just for a laugh). If you have time, take a short trip south to Nan Yue mountain, one of the 5 sacred mountains of China, with its impressive temples. The best thing about Changsha is the ordinary, everyday life - it isn't a flashy city, but my best times there were just doing ordinary stuff, meeting people, sharing their hospitality, eating spicy Changsha food and walking around with my wife. Chairman Mao studied in Changsha, so there are quite a few places to visit related to that, though I didn't get round to it.