The Silver Comet Trail can get a little crowded at the Mavell Rd trailhead since that's the closest point to Atlanta. Some people like to drive a little farther and get on the trail where it's a little less busy. I usually try to go in the morning when it's less crowded and then park at Mavell Rd anyways.

Some other areas than can be nice to bike in are: Piedmont Park, Stone Mountain, the neighborhoods around Decatur and Virginia-Highland. I've been doing a lot of group rides lately that go through various parts of Atlanta. But, the rides are fast (for me) and I regularly get dropped so, depending on your experience level, that type of ride may not be for you. I am trying to build my skills so I want the challenge of trying to keep up with the group.

I don't mountain bike (yet) so I can't help you with any trails for that. I hear there are some good ones.