I haven't really been to many places in the world that let you experience and be surrounded by remains of ancient civilizations. That's why going to Teotihuacan, the site of the ancient Pyramid of the Sun and Moon, was a particularly special experience. I strongly recommend going here if you are in Mexico City - try to go early in the morning though, right when it opens (I think 8am). There is really no one there at this time and its really surreal walking around by yourself hearing the echo of your own foot steps. You cant help but think about how thousands of people came together to build these structures with no technology whatsoever and would live, worship and play here. Wild.
There is an incredible amount of fascinating history behind this place that you can read about on the wikipedia page.