I think this is a good time of the year to find fun, inexpensive things to do:
-- Inman Park restaurant week runs from September 23 - 29th. Some of the top restaurants in Atlanta (Rathbun, Sotto, Sotto, etc.) are offering prix fixe meals at $15, $25 and $35 -- a substantial discount!
-- Go for dim sum on Buford Highway. (Google dim sum on yelp and you'll get a good list of places offering it)
-- Look up local festivals. This weekend there's the Decatur Book Fair, Marietta Art Fair and more. Free!!! Couple that with lunch in a good local restaurant -- lunch is cheaper than dinner -- and it's a great inexpensive way to spend the day.
-- Museums. The High, the History Center and I think Tellus offer free admission the first weekend of every month to people with Bank of America cards.)
-- Plan a picnic in Piedmont Park