The Caymans are my favorite island in the Caribbean for the beauty of the water, and I've been to all the main ones as far South as Martinique. I did the Stingray adventure a few years ago. Basically it's a sandbar where the tour guides capture a stingray and you can have your pic taken and they swim around you because they think you're going to feed them. I'd do it again; it was fun. I enjoyed being in the water (it's so clear there) and looking at all the various shades of blue and green.
Otherwise, Costa Maya is OK as is Cozumel. Your wife will probably hit you up for some jewelry in Mexico. The only thing I'd recommend is buying the Rum cakes and you can get vanilla extract there cheap, cheap, cheap (if one of you likes to bake).
I enjoy the days at sea with nothing to do but sail, and you got a couple of those. Very relaxing!
Have fun!!