I would suggest:
A deep water snorkel excursion, with a visit to Stingray City, in Grand Cayman would be good.
There are plenty of snorkeling and beach opportunities in Cozumel. A snorkel excursion out to Palancar or Columbia reefs is beautiful with plenty of fish to see.
In Costa Maya you can do an excursion to Coba or other Mayan ruins. This can be really interesting.
I am not sure about Roatan since I've never been there before.
No matter what you'll have plenty of choices for beaches and water activities on this itinerary.