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!
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.