Don't go to Isles de Mujers...very lame tourist trap....There is a place,similar to a national park in the US, but name escapes, Nevertheless, the concierge or front desk at the JW can refer you, as they referred my family to it.There are zip lines through and over a forest and river, swimming in the cenotes, fresh water swimming holes connected to underwater rivers, caves etc. All in all, it is an awesome day trip, good photos, and lot's of fun.
Was the location xcaet? It's a great plavce to spend a day if you are in the area. There is an underwater river tour that is really fun, No worries, they require you wear life jackets for the trip and wear a mask and snorkel so you can see underwater sights. Great for folks with a fear of diving. They also have dolphin encounters and lots of wonderful fun. The website is ;http://xcaret.com/ Don't Miss it !
This is one of my favorite areas to vist. Your sons might also enjoy the ferry ride from Playa DelCarmen to Cozumel where there is a interesting museum, lots of shops, resturants and where you can rent a jeep or a VW bug and run around the island in a day. Incredible beaches, sights and lots to see. Great diving as well.
If they are interested in history, stay on the mainland, rent a car and see Tulum, it's breathtaking, it's on the coast and relevant to history in the area. see :http://www.bing.com/places/search?q=Tulum%2c+Mexico&upgid=34119&FORM=ATRCCN
Xcaret is also on the way between Playa del Carmen and Akumal so renting a bug may be fun. The very best diving, including underwater caves, and an incredible fresh/salt water lagoon that is perfect for snorkeling is found at Akumal on the way to Tulum. You could vist the lagoon for an hour and then move on to Tulum. Coba is also in reach from Cancun but you might want to take a buss .It's way off the road and very primitive. If you want to see what an increduble site looks like BEFORE it gets uncovered, this site is the example. Many mounds are seen at this site and one or two are partially uncovered to give you a image of the what's here. You can get a soda , some trinkets and use the restrooms, but don't expect much else.