Since I was a kid, I had wanted to see the ruins of Teotihuacan, one of the largest and most significant archaeological sites in the western hemisphere. Unfortunately, we couldn’t climb the pyramids due to COVID restrictions, but it was still worth the trip. My favorite part was the Temple of the Feathered Serpent because of the decoration and sculptures.
I met a friend at the airport in the morning and we took an Uber to the site and then back to Mexico City in the afternoon. It took about an hour and cost about 30 usd each way. Buses are an option as well.
I would recommend getting started as early as you can to avoid the crowds and the heat, although neither seemed too bad to me on a Friday in the middle of March. We got there around 10am and spent a couple of hours walking around before lunch at La Gruta at 1pm.
La Gruta is a restaurant set in a cave within walking distance of the pyramids, and was a cool respite after walking around in the sun for a couple of hours. We had reservations, but I think we would have been fine without them.