If you have never been to Stingray City in Grand Cayman, you would probably enjoy that. We've been to GC three times and are going again in May and we are looking for something different to do this time. We've been to Stingray City twice and Seven Mile Beach once. The beach is beautiful, but was pretty crowded the day we went. We loved Stingray City! We've heard that snorkeling is great at Eden Rock (within walking distance of the dock), so we will probably do that this next time.
In Cozumel, both Chankanaab and Playa Sol are great places. If you want to snorkel, Chankanaab is better for that, but if you want a beach day, Playa Sol would be a better choice. There is lots to do there; we can't wait to get back there in May.