St. Simons is really laid back. My family goes to Sea Island (another barrier island which is in the same area). It's the kind of place where you lay out and drink. Well, those are my preferred activities. If you like golf, there are a lot of nice courses. My parents are actually going down next week and my mom will socialize and my dad will play golf.
For something historical, you could go to Jeckyll Island. There are a lot of historical mansions that were once owned by Northern millionaires (like the Rockefellers). You can tour all of them at once.
Cumberland Island is also interesting... it's where JFK Jr. got married. It's really pretty because it's all natural and pretty much uninhabited (well, there's one inn). There are also wild horses on the island which is kinda neat.