I have not been to Roatan so I can't help you there. In Belize I would do the cave tubing or take a trip over to Caulker Cay and do some snorkeling and have a nice beach day and grab some lunch.
If you've been to Cozumel then you know how good the snorkeling and diving can be on the reefs. There are also a number of beaches and bars that you can go to. You can also go to Chankanaab park and do the dolphin swim or snorkel in the protected reef area. There are full facilities at the park.
In Grand Cayman you can do Sting Ray city or you can do the submarine tour. I've heard mixed reviews about the turtle farm and I've been to Hell and wasn't impressed.
Take a look at the ships shore excursions to get an idea of what may interest you and either book the ship excursion or find one, like it, on your own.
Take care,
Mike