The Zocalo is HUGE! Go there when there is no one, go there when it is so crowded that there is not an inch left to stand in. It is impressive and one of the several hearts of Mexico City in my opinion. We stayed at a hotel next to it and our window looked out onto the Zocalo, which was quite nice as we did not plan this. There are groups of people who perform there (aztec dancers) and all sorts of merchants selling their goods. Head over to the Cathedral (went inside) and the Templo Mayor (missed it) for a great contrast of Tenochitlan then and now. Go early or stay late to watch the raising/taking down of the giant Mexican flag in the square. One day I hope to return and experience a band performing in the Zocalo and being squashed among the masses in the audience.