What are some fun things to do in Southern Georgia/N Florida for 22/23 year olds?

My sister lives right at the border of Georgia and Florida, about 30 minutes north of Jacksonville. I am going to visit her in August for about a week with a friend of mine. I've been several times before, but I've pretty much just been to the beach and have taken trips to St. Augustine and Daytona Beach. Any ideas on things to do around there? My sister has only lived there for 2 years (my brother-in-law is in the Air Force), so she doesn't have profound knowledge about the area.