You can pack so much stuff in your car plus you don't have to rent once you arrive!
We always make the drive part of the vacation. I find great stopping points with cool restaurants and parks to stretch your legs. And hit lots of redboxes along the way!
We drive from Indy so it's around 15 hours.
Here's a place in Chatanooga we stopped.
http://ridetheincline.com/
ate here...cool joint
http://www.mojoburrito.com/
Another time we stopped in Chatanooga at this park with a wooden carousel.
http://family.go.com/travel/things-to-do/tennessee/chattanooga/poi-282767-coolidge-park/
In Nashville, the Centennial park is wonderful!! A pretty lake to walk around, playground, cool train to look at and planes. Lots of food choices in Nashville.
http://www.nashville.gov/parks/locations/centennial/
In Marietta:
Take exit 269 B (Hwy 5) off I 75 towards Marietta
Follow the road for about 2.5 miles
The road will lead you directly to the square.
http://mariettasquare.com/
There is a nice, shady, grassy area for picnicking and a train themed play area.
Grab Pizza @ Marietta Pizza Company and spread out on the grass or at table on square. You could even call ahead and it will be waiting for you when you arrive! What a charming little town.