The beltline is a set of walking/biking paths created by repurposing a former rail corridor that essentially form a loop around Atlanta. While there are many fun areas to explore along it, the segment from Piedmont Park down through Krog Street Market is the closest to Midtown and probably has the most to see. Stroll around Piedmont Park and then head South down the beltline, with potential stops at Ponce City Market, Inman Park district, or Krog Street for food/shopping.
The Atlanta Botanical Garden is considered one of the best in the country, with a number of great floral collections and rotating art installations. With any weather luck, late September should be a lovely time to visit!
Ponce City Market (PCM to locals) is a bustling mixed-use development in a former Sears factory that has been heavily renovated/restored but retains much of its original architecture. The food hall contains stalls from various popular Atlanta eateries, and you can find a number of locally owned shops as well as name brand retailers. You also have the option of going up to the rooftop, which has one of the better views of the Atlanta skyline as well as carnival games and mini-golf.
Any baseball fans coming to town are in luck, as the Braves will be at home against the Pirates the weekend of the wedding, with games at 7:15pm Friday and 3:15pm Sunday (hopefully we will see you at the wedding on Saturday!). If going, it's worth getting there an hour or two early to stroll The Battery and get a bite to eat (we like Antico's).