JFYR, Coba is a bit primative and somewhat more unreconstructed than the more touristy Mayan ruins. Avoid the lake (about a dozen Caribbean crocs hang out there) and the poison trees (think poison oak), and take a good set of boots.
As to Muyil - looks cool but have never been.
Originally Posted by cozumelcruising
...just a small hand of ruins still can be climbed such as Coba and Muyil..