Teotihuacan was an ancient Mesoamerican city, now an archaeological site, open to tourists. It is situated around 25 miles from northeast of Mexico city. Ideally you should visit in the morning when there is less crowd and when the sun is not at its hottest. You will feel the heat even in the morning when you are walking around. So, bring a hat and wear trainers (sneakers). There is a lot of walking around, climbing uneven and steep steps, and working up quite a sweat. Great for Fitbit wearers!
I would highly recommend a trip to see this wonderful site. I would also recommend you visit Museo Nacional de Antropología in Mexico City, which is a huge, vastly interesting and impressive place with a lot of information about Teotihuacan culture and other Mesoamerican cultures.