madison may not quite be worthy of 1/2 a day but it's a charming historic town with many ante-bellum homes and would not be terribly out of your way to savannah. enjoy your trip!
Thanks, google maps show it would take an extra hour in driving time. I wasn't considering half a day for Madison but if we spent an hour or two we would probably be arriving in Savannah at about 4 pm and having just flown in from Australia the night before somewhat jetlagged. Madison sounds appealing but we will also be spending time in Charleston so I guess we'll have the opportunity to satisfy our appetite for antebellum houses?