Kinda depends what they consider "fun." Also how much they want to spend. Really if all they want a nice beach vacation and the usual sorts of activities that come with it, you can almost throw a dart at a Carribean map or the travel section of a Sunday paper - just pick a page with all the packages and charter deals. Aruba, Jamaica, St. Lucia, Barbados, USVI, BVI, Bahamas, Cayman, Puerto Rico come to mind as some of the usually easy to get to and mainstream places with plenty of nice places to stay. If they would like something less mainstream there's lots of other choices that will tend to be more laid back and probably not as many things to do. Or maybe they would like someplace like Belize, a bit more "exotic" compared to the usual suspects, spend part of the trip on the beach and part at one of the inland resorts, lots of interesting things to do there that aren't like all the other places. (Yeah I know not technically Carribean.)
Aruba has plenty of hotel choices, restaurants, shopping, casinos, nightlife, beach, beachy activities, snorkeling, a little sightseeing. It's fairly Americanized and somewhat touristy but that makes for more activities. But pretty nice and might fit the bill. Curacao would be good too, it's a little different and at least you feel like you've traveled to a foreign country.
I'm not a big all-inclusive fan at this point in my life, but maybe they would go for something like a Beaches or Sandals resort on whatever island. Or Couples, Grand Lido, etc.
