What kind of shopping does she want to do?
Among the Caribbean spots we've visitied, some (like St. Thomas and Curacao) have duty-free shopping: jewellry, watches, and perfume. Others (like Bonaire and St. Kitts) don't have that and don't really have handicrafts either -- just some tourist tchotchkes (at least as far as we saw). The Central American countries we've been to (Belize and Honduras, and Costa Rica) all have had *exactly* the same handicrafts: blankets, masks, wooden bowls, etc. The Riveria Maya and Cozumel have Mexican handicrafts: painted ceramics, mirrors, silver, etc. In all these places you can get t-shirts and such.
There are many nice islands for diving; your trip will probably be shaped by your fiancee's shopping requirements more than anything else.
Happy honeymoon!