Teotihuacan de Arista on the outskirts of Mexico City
One of the greatest visits in Mexico City, the mega-temples of the Sun and the Moon have once been the biggest citadel in the Americas. It takes around 45 minutes to an hour to reach the site in the northeast fringes of the metropolis. You can set up a fare with a cab to drive you there and wait while you explore before taking you back to the city or book an organized tour with a local operator recommended by your hotel concierge.
Go early, after a hearty breakfast, to have all the energy necessary to climb the steep steps of the tall Sun Pyramid and see the day slowly coming to a start from high above. Then walk towards the Moon Pyramid and take the best pictures of the day with the corridor of smaller pyramids.
I recommend some advance reading before you head there (or download some info on your phone to read while on site) because there is a lot to take in and the rituals and ways of life of the people who once lived there are very interesting. The visit takes around one hour.