In Mexican cities the main square called 'Zocalo'. Usually it is a nice park. Not in Mexico City. There it is a paved square with no else decoration than a huge Mexican flag in the middle. No plants, no shadow.
On the Zocalo we can find the lavishly decorated Cathedral and the 'Placio Nacional'. In the second one there are murals (huge paintings) on the wall depicting the life in Mexico before the Spanish and them influence in a stunning way.
Also visited the square around the Santo Domingo church and ate 'quesadilla' there.