I would strongly recommend the Caribbean, on a Princess cruise. I went in 2004 with my mother on the Sun Princess, which is a beautiful ship, not too big but with plenty to do and superb restaurants. Princess is primarily aimed for the more "senior" market, so for the 40-50 age group (which I am in myself) you can be sure that you would have an enjoyable time, and the cruise would be fairly child and adolescent-free.
I recommend the Caribbean as the voyages between each island take place overnight and there is only one or two full sea days. That means you will have roughly a whole day on each island. I strongly advise that you go to Barbados, which is simply beautiful.
Good luck with your choice.
Celebrity Cruises, RCI, Ocean Village are all cruise lines that I would recommen.