The Georgia Orienteering Club organizes o-meets througout the school year. It's loads of fun, though it is the one activity that will make me curse more than any other.
The Atlanta Scavenger Hunters Meetup creates brilliant hunts all over the city. Rather than simply trying to find a list of items, players look for pieces of information on signs, statues and landmarks to complete their cluebooks. I'm both a hunter and a designer with this group, and the benefits of membership are amazing (that last bit could be because I'm dating the club's founder).
The Georgia Sports Car Club organizes time-speed-distance road rallies in and around Atlanta. Not a race, TSD rallies are precision driving contests, penalizing players for being early just as much as being late.
A new curling club is organizing here in Atlanta, and I hope to take part this year.