Verbonc is nice that time of year...
Look at who is posting what and take it all with a grain of salt. If you think that the Grand Canyon would be the highlight of your trip then you might be interested in the Everglades (very south but worth it), Rainbow bridge, etc. Personally, I think the GC is a big hole in the ground; I much prefer something more urban.
Which leads me to point where I suggest that all the "Avoid Atlanta," people can bite me. If you are looking to tour the South your trip cannot be complete without spending time in the Big Peach. I'm not sure what the schedule is but you might be able to catch DragonCon as part of your trip--it's the largest con in the Southeast and one of the largest in the nation, behind but in the same league as Gencon and Origins. It's during the summer so it might work out for you.
The food is great--try Mary Macs in midtown for a taste of the old south (Ladies and Sons in Savannah is also great southern food).
Martinis and IMAX is a great gig they do at the Fernbank Museum of Natural History--have cocktails, dinner and dancing under the shadow of a T-Rex then go see an IMAX film.
The Atlanta Botanical Gardens, the history museum, the Margaret Mitchell House (of
Gone With The Wind fame) are all worth seeing. There is plenty of hiking and beautiful country to see (let me know if you want specifics).
The High Museum of Art is a nationally acclaimed museum and shows some really cool stuff-- I saw a pop art display that was just great. The CNN tour is pretty interesting and Cyclorama (this funky wrap around 348 ft. dioramma) is an unusual piece of Americana--if you like "Antique Roadshow" you'll probably like the Cyclorama (I thought it was well worth seeing, although it's not a whole day sort of thing).
And, what some other posters seem to miss is, Atlanta has great night life. The people are friendly and the clubs are varied and fun. There is something for whatever your style is. Don't miss it.
Of course, if you don't like big cities and would rather sit in the woods getting tick-bitten and watching dew form, try Celebrim's advice.
Seriously, you haven't seen the South till you've seen Atlanta.