If your daughter is old enough to stay up later, I highly recommend the Stone Mountain laser show. It's a really unique show at night against the mountain. During the day you can also climb the mountain (3 miles, I believe) or take a gondola to the top.
http://stonemountainpark.com/
Lake Lanier is really a nice lake but unless you want to rent a boat ($$) or swim, I don't know what to recommend. I grew up on our boat there.
Touring the Olympic park is really neat. Off the Olympic Park is World of Coke, Children's Museum and Aquarium but all of those cost money. There is also the Fernbank Museum in downtown
http://www.fernbankmuseum.org/. And for the older ones, the High Museum of Art
http://www.high.org/.
And of course, once you are downtown, if you want to eat at a tradition, go to The Varsity