Lagunas de Montebello is the great natural day trip from Comitán: a national park of more than 60 lakes set among pine and cloud forest near the Guatemala border. The appeal is colour, with water shifting through turquoise, green, blue, and darker shades depending on depth and light.
It is far enough from town that you should give it most of a day. Boats, short walks, caves, and viewpoints make it more than a quick photo stop.