In Cozumel, the best ruins experience is the Tulum/Xel-ha combination. However, these are quite a ways away and are an all day excursion. As there are so many more activities and things to see and do here than in Costa Maya - I would suggest your visit the ruins on your stop in Costa Maya. Visiting the ruins is the primary purpose for ships stopping at Costa Maya and other activities are somewhat limited. If you are looking for a fun light ruins experience - go to Chacchoben. It is only about a 4 hr. tour allowing you time to explore the port a bit or relax at the port pool/beach. Kohunlich is a larger site, but requires a much longer bus ride and takes about 7-8 hours.