Tepoztlan is a very nice and peaceful place to visit. It's famous for its spas, meditation, alternative medicine clinics and a very traditional city center. It is a small town so you don't need a car to travel within the city.
From Mexico City, you may take the metro to the Tasqueña station and follow the "autobuses" signs to the southern bus depot next door. The various bus companies display their destinations and times. Pullman de Morelos offers bus service every 30 minutes and a one-way ticket cost 84 pesos in 2010. The bus ride is comfortable, scenic and takes about an hour and a half. Upon arrival in Tepoztlan, the bus may drop you at a depot on the edge of town. Taxis will be waiting to take you to the main square for 40 pesos.
Tepoztlan is a small quaint town and is easily walkable. Most of the activity is centered around its main square and along the main avenue, Avenida 5 de mayo.
There is an archeological site at the top of an easy-hikeable mountain that is also famous for its "good vibrations". The pyramid, El Tepozteco, offers wonderful views of the surrounding area. Make sure to bring water with you. The hike to the top is free but it will cost 37 pesos (in 2010) to access the pyramid. At the top you will also encounter coati, also known as a tejón. These animals, members of the raccoon family, are curious and will attempt to open backpacks to find food.
Temazcal is an ancient mexican therapy that consists on a little igloo-shape room in which a group of people enter and medicinal plants are burned inside it and the place gets really warm. A guide asks people to sing ancient songs to make the experience more complete. Many hotels have this option either in groups or individual.
There are two camping places nearby: Camhomila and Meztitla, both nice and safe.