Muyil Ruins, Tulum

Small ruins with few (or no) tourists. Note that you can also hire a boat to tuor you around the lagoon where you can do some snorkeling.

http://www.tripadvisor.ca/Attraction_Review-g1575485-d313310-Reviews-Muyil-Quintana_Roo_Yucatan_Peninsula.html

https://www.locogringo.com/mexico/ways-to-play/mayan-ruins-archaeological-sites/muyil-ruins/