See what day the ship is in Grand Cayman. If it's on a Sunday you may as well pick an excursion because nothing is open.
We picked a fun day a the beach where you are bused to a nice beach, the driver has a mic and gives you a good narration of the tour and you get either a fruit punch or adult fruit punch at the beach. There was a bar, a restaurant as well. The only real downside is having to wait for a tender just for that excursion and having to wait for our bus. Really it was happy fun half day beach. The water was great, the beach was great and we had a good time. Undecided if I would do it again only because there were a lot of options for the day and I'd like to see more of the island.