Be sure to do the Wave Runner in Labadee, quite exillerating. It only takes about an hour so you'll have plenty of time for the beach. We liked the beach on the far end to the left, gives you a nice view of the ship.
Suggest Rum Point Beach on Grand Cayman, a dreamy caribbean beach and restaurant bar that 's not too crowded. Here's a LINK
to thier site. It takes about 50 min.s to get there by car/cab.. I heard the ferry, which was much quicker, is not running anymore but it makes the beach even less crowded. Also heard the Stingray thing was excellent.
RC has a Tulum Ruins/ Xel Ha tour on Cozumel that is excellent. Chakanab Marine Park is also supposed to be great if you like snorkeling. Xel Ha had some of the best snorkeling I've seen, tons of colorful fish everywhere! Only problem with that tour is that you'll wish you had more time at the Xel Ha place. Tulum was beautiful and educational.