Turneffe Island Lodge has been running a special lately. It's a private island comfortably accommodating 22 guests about 5 minutes away from what may be the best dive in Belize, The Elbow. How does your wife like to occupy her time? There's bone-fishing, sailing, kayaking, etc. My girlfriend snorkeled and lied in a hammock reading. We stayed in one of the villas and were quite happy with it. Meals are taken communally at three large tables in the dining room. The food was very good. If you want a liveaboard, the itineraries of the Aggressor and Peter Hughes boats are essentially the same, but I think Peter Hughes emailed me with a special recently. They will get you to a variety of great diving on the outer atolls.
I think the diving off of Little Cayman is as good as that on the outer atolls of Belize, but lacks the diversity in sites. I stayed at the Little Cayman Beach Resort. The accommodations were adequate, and the buffet meals were pretty good. The dive operation was pretty good as well, but ran a tight ship on a tight schedule, which lent an air of regimentation to the whole exercise. No solo diving, no dives over 50 mnutes. The dive staff were friendly and competent. There is absolutely nothing for your wife to do there.