How about a boat trip into the Sian Ka'an biosphere to float the Maya-built canals, see birds, manatees, non-captive dolphins, speed through the lakes, see the beach and some Maya structures, and more?
Also lots of cenotes to visit for swimming, each a bit different.
Coba ruins might be fun, even though not an expressed interest, since you can bicycle on the paths, climb a pyramid, see gators in the lake across the street, and visit the monkey park.