El Rey Ruins (Ruinas El Rey) are on the lagoon side of Blvd. Kukulcán at Km marker 17, a short distance past the Iberostar Cancún hotel (watch for the sign); take the stairway down to the entrance. The site, which is reached via a short trail, consists of a few temple platforms that once supported roofs.
El Rey is much smaller than the Yucatán's well-known archeological sites, and it's a nice, quiet change of pace from malls and rowdy nightspots. The ruins comprise 47 structures. The chief attraction here is not ruins but the many iguanas sunning themselves on the rocks. Fed by park guards, they're used to people and will come close to visitors.
Restrooms are on site. Wear a hat and sunscreen and bring water. Food is not allowed.