Go to the dry tortugas. There is a fast catamaran in Key West that will take you to the tortugas for the day. It will be important to check the departure schedule of your ship as this is about an 8 hour trip. It is great. It is a fast diesel powered cat that is wide it runs about 30 knots and the ride out is fascinating. At the Dry Tortugas is the historic Fort Jefferson. This is a pre civil war fort about 70 to 80 miles west of Key West. It is a very cool excursion. They have a marvelous beach and snorkeling spot very nice sand. They provide drinks and lunch. A tour of the fort which is a national park and then you can spend a couple of hours on your private island beach. If you have done the key west Duvall street scene this is a great change. I love Key West, but I would imagine that if you are not interested in people watching or bars with kids this would be the complete opposite of that experience.
I don't remember the cost of this... it has been about 3 years since we did this, but it is no worse than the regular excursions. I am sure if you did a search for key west and fort Jefferson you would hit this option. There are other ships that go there, but this is the only one that I know of that should be able to do it in the time frame that a cruise ship is in port.