The Atlanta to Stone Mountain trail is a multi-use trail developed by the PATH Foundation for recreational use by cyclists, rollerbladers, and other pedestrians. Some parts of the trail are bike lanes on the road while other parts are completely separate bike paths. A few sections run on the regular road, with no separate lane or path for cycling.
The trail starts on 5th street on the Georgia Tech campus and immediately heads east across the Downtown Connector and through town. It extends approximately 18 miles to its eastern terminus inside Stone Mountain Park, at the western gate. Actually, it ends just shy of the park but if you keep going along the same road, you will reach it soon. Entry into the park is free for cyclists and pedestrians and there's a nice bathroom/water fountain/food facility right inside the gate for you to use. It's also possible to ride right up until your tires are on stone though you'll have to get through some deep gravel first. The trail is marked with mile markers along its entire length and sometimes etiquette signs too. Directions and more information are availble from the PATH Foundation website.
The separate bike path sections tend to be either flat or gently rolling which allows for some high speeds. However, there are some serious hills. The most notable hills are the park sections towards the beginning (especially brutal on the way back) and the long hill by the DeKalb County Farmers' Market. It still makes for a very interesting ride through some nice scenery. There's plenty of stores and gas stations to stop along the way. Also, there are a lot of places to stop and turn around and still get a good ride even if you don't make it all the way to Stone Mountain.
I'm not particularly fast but I was able to make it the 18 miles to Stone Mountain in just under two hours and then back in just over two hours. This includes plenty of stops on both legs. More experienced riders should have no trouble averaging 12 or 15 miles per hour both ways. The return trip from Stone Mountain is more difficult. Traffic in some areas is a little thick but nothing major to worry about. In all, this is an excellent ride and you'll feel good after knocking down almost 40 miles of riding.