We could do it at the Chattahoochee River Park in Roswell. Its right on the river and they have a couple of very large picnic areas that are first come, first served. BBQ's there and plenty of room for whatever activities we want. I could head down early the day of to make sure we get a good spot and wait for the rest of the bunch to show up.
http://goo.gl/maps/wAzy
http://goo.gl/maps/wAzy