Things to consider
With a family of 5, if you want to go to the Caribbean the biggest consideration will be travel costs. If you can get free tickets via Airtran/SW, that would be great. Where does Airtran fly? That might be your best starting point. Then find out which of those destinations offer RCI timeshares, and then ask for input on those options.
We live in MA also, and have vacationed on a dozen or so Caribbean islands via timeshare exchanges: St. Thomas, St. John, St. Lucia, Aruba, Bonaire, Curacao, Antigua, Tobago, Barbados, Grand Cayman, Bahamas, Bermuda, as well as Cancun and Play del Carmen, MX. All enjoyable, but it depends on what you're looking for, ages of your children, budget, time of year you want to travel, etc.
Bermuda was one of our favorite vacations - short plane flight, beautiful beaches, very clean and safe, lots of family-friendly activities (Aquarium, Caves, etc) if you don't want to spend the entire time on the beach. Be aware that you can't rent a car on Bermuda, so you'll have to take taxis or the very-well run public buses to get around. We stayed at the St. Georges Club.
Bonaire was another of our favorites, primarily because my husband and I dive and the diving is spectacular. But our daughter participated in a kids club there while we dove, and they did some marvelous activities. They visited the salt works on the island and the donkey sanctuary, saw flamingos in the salt pans, did an intro to scuba class in the pool, and more. There are few beaches on Bonaire, so if miles of soft sand are your goal, this would not be the place for you. Exchanges are hard to come by here. We stayed at Buddy Dive.
Grand Cayman is another place we liked. Again, there are some interesting family activities if you want to get off the beach for a day. Nice beaches, great diving and snorkeling, fair amount of timeshare availability. Clean and safe, too. We stayed at Morritts and Morritts Grand.
We fly primarily AA. Jet Blue has started flying into some Caribbean destinations, and the competition has driven down AA prices, which is great. Jet Blue's presence in Boston has increased significantly, so be sure to look into them for future travel.
There are lots of great timeshare destinations in the US that I think you and your family will enjoy as well - some of them already mentioned by others. I'll add Vancouver Island, Yellowstone, So. California, Alexandria, VA (for visiting D. C.) and if you're skiiers, lots of great skiing out west.
Now, when you decide you're ready to start thinking about Europe, let me know. I have some favorites there, too.
Hope this has helped.
-Corinne