I would recommend the clear cayak excursion. You cayak out to the reef where you snorkel for a few hours. Also, the reef runner excursion. This includes some more snorkeling.
As for motion sickness if you go to your doctor to get a prescription you can go to the pharmacy and pick up small patches that go behind your ear. Put them on 3 hours before boarding the ship and you should have no problem. They last for 3 days so you may want to pick up a couple for each person. They can be a bit expensive so I would recomment one per person plus 2 extras for whever may need it.