Cumberland island is cool if you are able to devote a whole day and walk a lot... It is a national seashore island and they only allow something like 200 visitors on the island each day. It is where JFK Jr. was married. There are ruins of old estates and fortresses on the island. A lot of wild horses and wildlife,,,but there are no outsider cars allowed.... it is all walking in the park area. Not sure about the residents area or if visitors are even allowed there. There are no stores so you have to pack a picnic lunch and drinks if you want them.
You have to ride a ferry to the island out of St. Mary's GA . The ferry ride itself is real nice, about a 45 minute trip each way. Dolphins swam beside the ferry and jumped beside us when we went.
http://www.stmaryswelcome.com/cumberland.html
Savannah GA is real nice too,,,very historic and quaint.
Guana River State park in Florida near Ponte Vedra Beach is nice as well as Fernandina Beach, FL. St. Augustine FL is a not-to-miss day trip also. Very historic, quaint and fun.
Florida has many natural springs too... I am not sure which ones are off of I95 though... I know there is a really nice one near Apopka FL (Wekiva Springs) which is about 30-45 min north of Orlando. The pictures do not do the Florida springs justice... never have you seen clearer water. It is very cold though,,,,around 72 degrees all year long. Some of the springs have manatee habitats and viewing areas.
http://underwaterflorida.homestead.com/springs.html
Have fun!