My favorite ones are the one at Turner's Corner (12-15 miles north of Cleveland) and at Moccasin Creek State Park (at Lake Burton).
We live a mile outside of Helen, and will run to Moccasin Creek when we can. It's about 20 - 30 minutes from Helen, and is on the shore of Lake Burton. There's a fish hachery where the kids (and seniors, if I remember correctly) can fish. All the sites are level, or reasonably close.
I haven't stayed at Turner's Corner in several years, but before we moved to the area, we were there every couple of weekends. It's at the corner of Hwy 129 & Hwy 60. Some of the sites are on the creek, and others are in the middle of an open area. Sites are level, but whatever you do, don't speed through the lot, the owners will jump on a golf cart & hunt you down (or used to).
We did the popup thing for a while, but now I get to do the cheap trailer setup - I borrow my parent's camper. With it, I have to borrow their truck /forums/images/graemlins/cool.gif cuz we haven't figured a way to set the camper to work with his truck (1/2 ton Dodge 2wd) and my truck (3/4 ton Dodge 4wd).
There are a million campgrounds around, we have a tendency to find one we like and stick with it. Let me know if you have one you're interested in, maybe I can get you a drive by description.
Jeff