An easy day trip from Mexico City is Puebla. This is the fourth largest city in Mexico and only 2 hours away by bus.
There are several bus companies running buses from TAPO station to Puebla (CAPU). There’s no need to book a ticket in advance. Buses run every 20 minutes and it’s easy to purchase at ticket 15 minutes before the bus departs. Give yourself some extra time though as the bus station is large. I took the bus company Estrella Roja as they offered a first class express bus for 190 pesos each way. I decided the spurge on a bus with air conditioning.
When you arrived at the bus station in Puebla, take the exit towards local transportation. The local buses are lined up outside the bus station. They run frequently and are by different companies. Find a bus that says Zocalo and pay the 6 pesos fare. This will take you to the main tourist area. Make sure you have an offline map saved of the city and pin a main site like the cathedral or main square. You can track where you are on the bus and pick a stop close to your destination as there aren’t laid out bus routes (at least none advertised).
Once you’ve found Zocalo, wander from there. The cathedral in the main square is worth a visit as you’re there. I recommend visiting the church of Santo Domingo where the chapel is outstanding. There’s even a free guided tour offered in English for those like me looking for details about the frescos and idols.
I could easily spend a few hours wandering taking photos as the area is beautiful with its colourful buildings and friendly people. The area is a UNESCO world heritage site if you’re interested in checking that off your list.