In Cozumel, you may want to get a cab over to Chanakaab Park, huge beach and good snorkeling, places to eat and walk around. It is not that far from the pier, but not walkable...Depending where you dock in Cozumel, you may be right in town or farther away. There are plenty of cabs that go back and forth along the main street and Chanakaab is also off the same road, just the other direction.
In Jamaica you can always walk up Dunns River falls, not sure how far away your port is from there.
You can get some good ideas by going through the list of excursions that the cruiselines offer and see what interests you and go from there, some you can do on your own.