Just across from Playa Delfines [1], Ruinas El Rey (Blvd. Kukulcán Km. 17.5, 8 a.m.–5 p.m. daily, US$3.50) consists of several platforms, two plazas, and a small temple and pyramid, all arranged along an ancient 500-meter (1,640-foot) roadway. The ruins get their name (Ruins of the King) from a skeleton found during excavation and believed to be that of a king.
The ruins date from the late Post Classic period (A.D. 1200–1400); signage is available in English and Spanish. Last visitors admitted at 4:30 p.m.
The ruins are home to literally hundreds of iguanas, some quite beefy, which makes a visit here all the more interesting.
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun/sights/zona-hotelera-beaches