Key West has some interesting history. There is a multi part documentary on Hemmingway and Key West features in part. The Hemmingway House is still there. Maritime Museum. If you want to go in October you can always time it for Fantasy Fest which is a spectacle unto itself.

Not sure how much of the east coast of Florida you have seen.

Personally I would suggest Kennedy Space Center for a full day. I love going there and it still amazes me that the rocket ships are not replicas but actually went into space.

Downtown Savanah is nice with a lot of history.

After that it just depends on what you mean by "Carolinas" and what your interests are.