Tour Guide to Teotihuacan in Mexico City

The TripAdvisor "Things to do" pull-down menu lists a number of day tour providers specializing in trips outside the city, and we found their reviews to be pretty reliable in trying to choose one ourselves.
Am sure you can find options for private tour operators if that's your preference.
Might add that a day trip out to the Monarch Butterfly Sanctuary in the neighboring state of Michoacan was a particular highlight for us. The little guys (millions of them) winter over in the forest there - you'd need to Google the exact months they're in residence, but it's a spectacular show.

Thanks Robert, I'll check that out!