Chankanaab is my favorite snorkel/beach spot in Cozumel. It's easy to get to, and there's a lot to do there besides snorkel. We did our swim with the dolphin there and it was fabulous!
Jamaica is a beautiful island, but I totally agree with Jim, stick with a ship's sponsored tour there. Dunns River Falls is fantastic and I think you all would be able to climb the falls. They've added a walk way beside the falls now, so if you don't feel comfortable climbing the rocks, you could walk up the side.
Labadee is really like a private island, so that will definitely be a beach day. They'll have a bar-b-que and the ship's crew will be there running the show. They have zip lining there, but it fills up fast, so I'd book that on-line if it interests you.
I'm so glad you're finally getting to go on your cruise! Just be warned, cruising is addictive:)