I'm looking at taking a trip down to Florida in late March to visit the mother-in-law in St. Pete, watch some baseball, and spend a day or two at Disney/Universal. Everything is very tentative right now, and we've got to do a little bean counting to see if we can really afford to take the trip, but here's what I've laid out so far.
We'd leave Friday afternoon, ideally right after school is out and travel south through Illinois, possibly getting as far as Paducah, getting a motel after stopping probably around midnight or so.
The next day, we'd continue south, on I-65/US-231 through Alabama, and possibly camp for the night in the Apalachacola National Forest near Tallahassee, and finish the trip on Sunday morning. I'd probably take a quick detour through Land Between the Lakes, but I don't have much else in mind this day.
We'd stay in Florida until Friday morning, when we'd head back north to Atlanta. Some possible stops this day would be Andersonville, or several of the Atlanta ideas that Peter Thody recommended. Probably would be camping near Atlanta this night.
Saturday would be a pretty fun day, possibly spending a little more time in Atlanta and then heading up to Smokey Mountain NP (I'd also get to cross off North Carolina from my list, as it is one of the 2.5 of the lower 48 states that I have not yet been to). Probably would spend the night camping in the park, and then make a 700 mile sprint towards home on Sunday.
The south is really the one part of the country I really haven't spent much time, so I'd certainly be open to other suggestion or stopping places along the way.