Let me go ahead and start of with the obvious. There is the Bridge Run. It's kind of a big deal. Over 40,000 people are going to travel to the Holy City for a once in a lifetime chance to run over a bridge that people run across every day. It's magical, to say the least, but it also only takes up a couple of hours of your weekend. So then what? Here are some ideas to help you make the most of the weekend.
Post-run Afterparties - King Street and the Market will be packed with people enjoying many of the Bridge Run's after parties. If you're finishing the run or just don't mind dealing with the traffic on Saturday, you will definitely have fun downtown. My suggestion: Oku on King St is a great spot to get some refreshing cocktails and some of the best sushi in town. It's also close to Marion Square, so you'll be right in the center of the madness.
Mini-Cup - Basically, you take the Carolina Cup, remove the horses, and add liquor. It's a recipe for success. Grab your best pastels or sundresses and barhop your way to glory (or defeat). If you find yourself wondering why people in Charleston are dressed so "plantation" style, this is why. Don't ask questions. Just do it. Get some friends, dress up, and go enjoy Southern charm at its finest.
The Harbor - Don't just stare longingly at the water. Get out on it! You don't even have to get wet. Boating season has started, and there really isn't much that can give you the same sense of satisfation as a good day out on the water. Heck, we do this for a living and it STILL never gets old! There are lots of charters and cruises that launch right out of downtown, so you don't even have to travel far.
Cajun Fest - New Orleans and Charleston have a lot in common already, but once a year we join forces for an amazing day outdoors at the Cajun Festival. It takes place Sunday in James Island County Park, a quick 10-15 minutes from downtown, and features all of the sounds and flavors that make swamp/marsh living what it is. Dance to the brass band blues and eat more gumbo and crawfish than you should. New Orleans and Charleston were separated at birth, and this is one reunion that you don't want to miss. Check out this article for more info.
If you're still alive after all of this, I salute you. This weekend requires preparation even if you're NOT running over a bridge, but the reward is well worth the price of admission.