Best free things to do in Atlanta?

More free stuff is the Monetary Museum at the Federal Reserve Bank of Atlanta, the view from the Sun Dial and hiking Kennesaw Mountain National Battlefield Park. 

Vortex Bar & Grill in Midtown is great burger place. Lots of free festivals and hiking trails at Roswell Mill Club. Walking through Piedmont Park in Midtown, Booth Western Art Museum in Cartersville if you want a little drive, Roswell alive at 5 (http://www.aliveafterfiveroswell.com/) is a lot of fun. The Varsity is greasy and cheap. Bulloch Hall is a small museum in Roswell.

The Atlanta Botanical Garden near Piedmont Park  is awesome. It's not inexpensive, but worth the entrance fee. Or, you can wander around Piedmont Park for free and grab a drink or some food at cute restaurants near the park (The High Museum of Art is within walking distance of the park as well). Virginia Highlands is also a great neighborhood for eating and shopping.

One of the many breweries: SweetWater Brewing Company or Monday Night Brewing are decent.

If you're into something a bit risqué visit the Clermont Lounge 

Stone Mountain Park is an easy hiking spot right outside if the city.

The Georgia Aquarium and World of Coca-Cola are very touristy, only visit if you like crowds. 

Little Five Points is a fun place to spend an afternoon people watching.

Piedmont Park was great for a walk or a bike ride. First Thursdays Art Walk Downtown is so cool (http://www.atlantadowntown.com/fun/first-thursdays-art-walk). Tons of open art galleries, don’t miss Luckie Street Gallery & Studios and/or the Apex Museum both of which always have interesting exhibits. The Fernbank Science Center was really interesting as was The Carter Center which you can walk through. Check out the Mall of Georgia for free events, too. Centennial Olympic Park Dr NW was kind of cool too.

Weather permitting try the Beltline Trail, it runs through many different neighborhoods.  The Sweet Auburn Festival is in May and there are others found on this events calendar - http://www.events12.com/atlanta/may/

You mentioned free, however Atlanta also has their version of the CityPass.  It is starting to warm up ... take water with you as it is cheaper than buying it along the way.

Little 5 Points is a nice and unusual shopping area.  It's near Inman Park, Edgewood, Candler Park

Hike Stone Mountain Park.  But there is a fee to get in the park