Sara, that is a great itineray. You will really enjoy it. We are on the Dream 2 times this month and really looking forward to sailing her.
Having been to a few of these places in Cozumel I would do the Isla Pasion Twister excursion. They take you to this beautiful island with a great beach and lounge chairs and hammocks and it has a good buffet and a limited open bar...it was a great, relaxing day when we went.
In Belize we did the Barrier Reef snorkeling and it was really nice. The water there is gorgeous. Other than that I would do the cave tubing but that is something you have to book on your own but TONS of fun.
Costa Maya the Dune Buggies are fun, we try to do these any chance we get.
Never been to Roatan so can't help you there. Have a wonderful time