A chain of three cenotes, an hour by bus or car from Merida, in Yucatan.
My suggestion
If you have a limited budget, you can go to Homun which is nearby, and you can bathe in the cenotes there (cheaply and alone), one of which is actually in someone's garden!
Keen to discover the northern region of Mexico? The Nuevo León, as well as Coahuila are two regions housing cities to see, when going on a visit to the north of Mexico. In fact, Monterrey and Saltillo have several attractions.
The north and south are two different worlds, not only in terms of their landscape but also their economic structures. On this one strip of land fierce industrialization contrasts with a laid back tourist lifestyle.
I guarantee that hopping on the El Chepe train will be a unique adventure in the north-west of Mexico. As the kilometres go by, incredible landscapes will pass before your eyes, don't miss the train!