A good long day trip out of Atlanta (driving about 240 miles):

Take I-75 south from Atlanta (alternately, use 19/41)

In Andersonville (just north of Americus) is the Andersonville Nat'l Historic Site/Cemetery. This was a Civil War prison. Interesting museum.

In Americus (about 10 miles further) is Habitat for Humanity HQ. Not sure if they've got a museum.

In Plains (about 10 miles further) is Jimmy Carter Natl Historic Site.

Then, go further west on GA 27 to GA 41, then north to Warm Springs to the Little White House State Historic Site (where FDR spent winters and recuperated from polio).

Interesting change in terrain in this area...you go from the (semi) coastal plains to suddenly climbing 500 feet onto the plateau.

Also in the area is Callaway Gardens. Went once when I lived there, but didn't impress me that much (however I grew up in St. Louis and grew up going often to the Missouri Botanical Garden www.mobot.org and like that much better)

Enjoy.