Sian Ka'an reserve is mostly wilderness and wetlands. Access to beaches are from Punta Allen, and its not all that easy to get to.
http://www.travelpod.com/s/punta+allenWatch out as this site is mixed up with some blogs from Punta Arenas, Chile.
Akumal - not really worth it, you will see much better along the way.
Xcaret - A disneyesque nature park. My grand children loved it. It's expensive, but if you have the time and finances, you might give it a try. Don't cry if you give it a miss.
http://www.travelpod.com/s/Xcaret Xel Ha - Interesting rock pools for snorkelling. It's just off the route, so it can't do any harm visiting it. To my mind, it's the best of the three, and it won't break the bank.
http://www.travelpod.com/s/Xel-ha
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