I second Stingray City. It was awesome. I also talked to one man who went on a submarine down 500 feet on the reef wall (which is 2000 feet) near Stingray City. It was a 3 man submersible and he said it was the best thing he ever did. Not sure of the name.
The Turtle farm was cute, but not a life changing event so to speak. If you have kids, they might like it. You do have an opportunity to hold and touch the turtles.
Enjoy your trip. We have been back for a week now...and I'm so bummed out about it.