There are several beautiful singletrack trails spread throughout the park. They are rather rocky and rooty, and these are mostly intended for mountain biking, so expect significant bike traffic.
From Atlanta, GA 400 N - Take the Holcomb Bridge exit, turn right (east) off the exit, go 1/4 mile and turn left on Old Alabama Road (at traffic light). Parking about 1/4 mile on the left, gravel parking lot under the power lines.
http://www.tripleblaze.com/trail.php?c=3&i=212