Something for everybody here. If the weather co-operates, you'll have a great time. The beach chairs do not cost anything. Locals will try to "help" you find a spot to your liking to set up chairs for you. You do not need to use their help and you are not obligated to tip them. RCI tells you they would prefer you not pay them. We didn't know that and we did tip.
We choose a catamaran / snorkel excursion. They drive you to Montego Bay which takes about 30 minutes or so to get to. As expected the snorkeling was so so but the catamaran ride was very fun. The crew spent time showing and talking about the local waters etc which was interesting to us. The best part was the time spent under sail on the way back (or was it the rum drinks). The combination was great.
Since we hadn't seen any Mayan ruins on previous Cozumel stops, we went this time. If you have no strong interest in seeing ruins, don't go. It is a long day starting off with a motion sickness testing ferry ride to Playa Del Carmen. Remember you have to take that Ferry back too. Many people suffered. Thankfully we didn't and had a decent day even though it was a bit cool. This prevented the beach break, after seeing the ruins, from being as enjoyable as it could have been.