The most accessible Maya site within the Sian Ka’an reserve  is Muyil (Hwy. 307, 25 kilometers/15.5 miles south of Tulum, 8 a.m.–5 p.m. daily, US$3), on the western edge of the park. Also known as Chunyaxché, it is one of the oldest archaeological sites in the Maya world, dating back to 300 B.C. and occupied continuously through the conquest.
It’s believed to have been primarily a seaport, perched on a limestone shelf near the edge of Laguna Muyil; it is connected to the Caribbean via a canal system that was constructed by ancient Maya traders and still exists today.
Only a small portion of the city has been excavated, so it makes for a relatively quick visit. There are six main structures ranging from two-meter-high (6.6-foot) platforms to the impressive Castillo. At 17 meters (51 feet), it is one of the tallest structures on the peninsula’s Caribbean coast. The Castillo is topped with a unique solid round masonry turret from which the waters of the Caribbean Sea can be seen. Unfortunately, climbing to the top is prohibited.
A sacbé (raised stone road) runs about a half kilometer (0.75 mile) from the center of the site to the edge of the Laguna Muyil. Part of this sacbé is on private property, however, so if you want to access the lagoon from the ruins—you also can get to it by car—there is an additional charge of US$3 per person. Along the way, there is a lookout tower with views over Sian Ka’an to the Caribbean.
Once you arrive at the water’s edge, it’s possible to take a boat tour (US$33 per person) that crosses both Muyil and Chunyaxche Lagoons, which are connected by a canal that was carved by the ancient Maya in order to reach the ocean.
It’s a pleasant way to enjoy the water, and you’ll also get a view of several otherwise inaccessible ruins along the lagoons’ edges and through the mangroves, with the final stop being Xlapak ruins, a small site thought to have been a trading post.
If arriving by car, look for signs to Muyil Lagoon on Highway 307, just south of the similarly named archaeological site. More thorough tours of this part of Sian Ka’an can be booked in Tulum .