Honestly, not going to the archeology sites in the Yucatan is such a shame. Would they not at least want to do Tulum? It's quite small, and there's a lovely beach below the cliff that would be wonderful for sunbathing.
Our group also had a great day by renting a water taxi that took us across to the island, and allowed us to go snorkling at various spots, as well as checking out the Sea Turtle Rescue, and having lunch and drinks. It wasn't too expensive for an all day tour. However, we did rent it on the spot, so I'm not sure how to go about arranging it from outside Cancun. Maybe the hotel could help arrange something?
Have they said what they want to do while they're there? It sounds like they're not into a lot of things you go to Cancun to do. If it's primarily "lie on the beach and relax," perhaps you could get them some nice beach towels and things to go with that?