Honestly, It really depends on what you want to do. We were in Grand Cayman back in '19 on the fantasy and enjoyed walking around the port area for a few hours. The Margaritaville is a few blocks from the port and has a nice pool and slides. it is moderately priced but the food and drinks were great.
Most people I know prefer to set up their own snorkeling excursions. There are options in the port area for this if you want to meet a tour. Otherwise, lots of the resorts offer day passes with a cabana/room rental option which is very appealing if you prefer not to sit in the sun all day. Whatever you do, if you end up going on your own make sure to leave 30 min - an Hour for the tender once you arrive at the port to make sure you are back on the ship on time.
Enjoy your cruise!