The botanical gardens for sure. I was there about a week ago and the roses were in full bloom. They were swapping a lot of stuff out too, so it should be extra special now. There is a very crazy sculptural exhibit on now.

GGG

There is always some crazy sculptural exhibit there!

I can recommend the Fernbank Museum. It has a huge rose garden, about 10 times the size of the rose garden at the Botanical gardens. You don't even have to pay unless you want to go into the museum.

http://www.fernbank.edu/museum/

If you have an interest in Native Plants, the GA Perimeter College/Decatur Campus has a Native Plant Botanical Garden that is open to the public. Don't know if they have a guide available during weekends, but every plant is labeled, so a self directed tour is possible. The garden is accessible from the Panthersville Rd parking area, just beyond the Campus buildings.
On the website below, click on Directions for map and driving directios from I-285.
Enjoy your trip!
Rb

Here is a link that might be useful: Georgia Perimeter College/Decatur