One of the tours I took and you should too is a tour that leaves from San Cristóbal de Las Casas in the morning to get to the National Park Lakes of Montebello. The park consists of 59 lakes, some of them very small, some bigger, some very contaminated like the lake Encantada with weird green-yellow color of the water where is prohibited to swim. The three main lakes to watch here are the Ensueño, Encantada and Esmeralda. You have to go down the steps to visit all three of them. The biggest lake in the national park is the lake Tziscao, which is of clean green water. At Tziscao you can spend your time hiking, camping or doing some sports such as mountain bike. I visited a total of 6 lakes here but, if you have time you can visit many more.