CDMX
San Juan Teotihuacán, State of Mexico
Google Maps Directions Website
Outside Mexico City, in the State of Mexico — a day trip rather than an afternoon.
No reservation is required; you can buy at the gate. Buying ahead on the INAH portal (boletos.inah.gob.mx) mainly saves queue time, and foreign cards are often rejected there, so don't count on it working.
TODO: how we're getting there (tour, bus from Terminal Norte, or a driver) and what time we want to leave.