Coba is in between Valladolid and Tulum, and probably two hours or less direct from Cancun. The town sits at the side of a croc-filled lake and there are adventure activities such as a zip line ride over said croc-filled lake. It has a smattering of typical tourist restaurants and shops.
The ruins are spread out but bicycles and pedicabs are available, and walking the jungle paths is a pleasant bonus that the other sites lack. Nohoch Mul (Big Mound a.k.a. the Great Pyramid) can be ascended unlike its counterpart at Chichen Itza. The other buildings are not as extensive as Chichen Itza, but have good variety. With better tourist facilities and the lakeside setting, Coba is a good family-friendly alternative to Chichen Itza.
Muyil on is the edge of Sian Ka’an Biosphere Reserve, 16 km south of Tulum. Tourists based at the many resorts in Tulum can use it as a diversion from the over-touristed center. It is small in scale and loaded with mosquitoes. Its crumbling state is charming but does not take advantage of its natural setting, looking the same as the inland sites. Only worth the effort for those in Tulum who do not want to drive farther. There are reports of day tours in Sian Ka’an run by Muyil locals, which would be a good outing. Check local agencies.