There are boat and Seadoo rentals in Sea Pines Plantation.
There's the night life, if you're old enough to drink and like to. I don't know what bars are there anymore, but ask the locals, they know the best places. They used to allow 18 and up in the dancing places, not sure if they still do though.
You can take a day trip to Savannah. It's one of the most beautiful towns in the South and only 30 minutes away. You can sit in the same square on a bench just like Forest Gump sat on (the original, and about 200 replacements have been stolen).
Now that I think about it, all I did when I lived there was get drunk and hang out at the beach... then go home, shower and drink all night barhopping.
Man, I don't think I could do that anymore. 😀