I am from Charleston and currently live in Savannah. They are 2 hours apart and the drive between them is scenic - through the marshes. In Charleston, you have the historic downtown with good hotels, B&B Inns and restaurants. In January, you could take a carriage or walking tour, see house museums, visit the aquarium, take a boat out to Fort Sumter or go visit Fort Moultrie on Sullivan's Island. Then there are plantations that can be visited. Boone Hall Plantation is in Mt. Pleasant and Drayton Hall, Magnolia Hall and Middleton Place Gardens are "West of the Ashley." If you enjoy such things, you can go tour the Yorktown Aircraft Carrier at Patriots Point, and you can also walk over the big bridge that separates downtown Charleston from Mt. Pleasant. You can also go see the Hunley civil war era submarine which has been recovered. If you prefer the beach, you can get favorable winter rates at Kiawah Island and rent bikes to ride around, walk on the beach, play tennis or go golfing. The inn there has a large indoor pool. Charleston can keep you entertained for a week, but I would at least give it 4 nights.
En route to Savannah, a stop in Beaufort is fun. We have ridden bicycles all over, stopping for a nice lunch or to tour a church and cemetary. Off of Beaufort, there is Hunting Island State Park which is lovely and you can climb the lighthouse there. Hilton Head Island is also between, and in January, you can usually enjoy walking on the beach. From Hilton Head Island, you can get a boat to Daufauski Island which is very pretty to tour or stay.
Once in Savannah, the activities are similar to those in Charleston, minus the plantations - carriage rides, house tours, walking, biking, some museums. Savannah is particularly nice to walk due to it's grid-layout with wooded squares. You can go out to Fort Pulaski - masonry, or drive down to Richmond Hill to see for Fort McAllister - earthen. Out near the interstate, there is the Mighty Eighth Airforce Museum.
South of Savannah I can also recommend St. Simons Island and Jekyll Island. These two can be reached by car and are worth the visit. St. Simon's Island has good resorts and also Fort Frederica National Monument which you can visit. Jekyll Island was a "millionaires club" a century and more ago and their "cottages" are available to tour. You can either stay in the hotel which was their clubhouse, or have a meal there which is fun. Biking on Jekyll is especially fun as there are lots of wooded paths, and low traffic roads.
I think I could write a similar glowing recommendation for driving the Florida Keys, starting with Everglades National Park and working your way down. I have done it several times, and it's one of my favorites. It will be more reliably warm than Charleston and Savannah in January.