Best fun we've ever had was on the haunted hearse tour! Drove around to various pubs, sitting in chairs bolted to a convertible hearse with a canvas cover, shouting at random passers-by. Oh, and we saw some haunted places, I think.
Food in Savannah is tough, especially coming from Charleston. There are so many restaurants that cater to tourists that they don't have to be good, but just have a good reputation. In Charleston, the restaurant culture is so strong that a bad restaurant just doesn't survive (Hyman's is the exception) the withering glare of locals.
I'd probably say Pink House or Mrs. Wilkes to be the sort of thing you are looking for on Thanksgiving. Even though heavily touristy, they still maintain pretty high food standards. Elizabeth's on 37th is deservedly famous, but may not have the buffet you want. I'd eat there over the others in a heartbeat.