Tepotzotlán 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 40 minutes and a one-way ticket cost 120 pesos in May 2016. 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. Walking to the center will take 20-30 minutes or so.
From Cuernavaca, buses run along the Avenida Morelos/Avenida Emiliano Zapata (northwards direction) which are the same street but the name changes north of Bajada de la Pradera. Buses stop at set stops along the route to pick up passengers but they will also slow down and open their doors if you flag them anywhere along the road. The San Jose Catholic Church is a reliable stop for this service. The bus runs from 640-2100 on weekdays, 0700-2100 Saturdays and 0800-2100 Sundays every 15 minutes. The bus name is Ometeochtl but will generally display 'Tepoztlan' as well. ~40 pesos for the, 40 minutes to Tepoztlan station, 60 minutes to the centre, ride.
To return to Cuernavaca, simply follow Del Tepozteco south (it's the same road that leads to the trailhead for the ruins) for twenty minutes back to the bus station.
Going back to Mexico city, it is possible to hitchhike by the toll booths at the edge of town. You can also come back to the Bus Depot where you got down in Tepoztlan and take the bus back to Mexico City.
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 a hikeable mountain (1.3 miles of steep stone stairs, around 45min-1hr climb at a good pace) that is also famous for its "good vibrations". Bring water, wear closed-toed shoes, use a backpack to carry your things, take breaks while climbing and be especially careful if you have any medical conditions. The climb is tough if you are out of shape and/or not used to the altitude! It's definitely worth the hike to reach the pyramid, El Tepozteco, and the wonderful views of the surrounding area. The site may be reached by walking down Avenida 5 de Mayo (in the direction away from where you entered the town). Continue until the street ends to the steps which mark the beginning of the trail. The hike to the top is free but it will cost 47 pesos 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.
There is also a beautiful former convent called Dominico de la Navidad. It has been restored and entrance is free. It is located right off the main square.
If you are curious about it, the locals claim UFO sightings are common, wich mostly just adds to the "new age" vibe of the town. Ask about the "ovni" to hear strange tales.
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.
Tepoznieves has multiple locations around town (Av. 5 de mayo, 21) and their ice cream is famous. They may have up to 200 flavors available ranging from the pedestrian chocolate to more exotic rose petal, figs & mescal or carrot with chili.
La Posada de Tepoztec is located two blocks above the main square. In addition to offering beautiful (though pricey) lodging, the restaurant serves delicious local cuisine. Sit on the serene patio to enjoy the mountain and town views. Their Chicken Mole is delicious.
There are two camping places nearby: Camhomila ($245 camping per person, wow) and Meztitla (Campo Escuela Meztitla, beautiful, large grounds with amazing views of the mountains, $140 camping per person, $200 for renting tent), both nice and safe.
Posada Ali is nice place with a good view of the cliffs. It costs around $500 on weekdays (prices seem to vary).
Sleeping in Tepoztlan can be noisy at times (in December 2010, the curch bells seemed to ring on the half hour all night).
A taxi from the town center to the bus terminal cost only $30 pesos in February 2014. The bus ride back with Pullman de Morelos costs $104 pesos and takes a bit more than an hour to get to the Autobuses del Sur terminal in Mexico City. Buses ran approximately every hour, on the half hour, in March 2013 but check the posted schedule when you come into town and you won't have a problem getting home.