Where is a good beach vacation spot for teens?

My friends and I want to take our Senior trip together to the beach. We live in Tennessee, and we don’t want to have to travel far. We were thinking maybe Florida. We want to go somewhere that isn’t overly crowded, but still has a lot of things for teens to do. Any suggestions?

I live in Florida – South Beach is nice but very expensive. But there a lot of good deals on hotels because of the economy. Food and bars are expensive.