One of my quick little weekend day adventures while in Mexico City was a brief visit to the Castle of Chapultepec, a royal castle that been used as everything from a royal residence, to a military academy, to a museum. It’s easy to get to on public transportation (it has its own metro stop) and is a quick little adventure for a morning or afternoon in Mexico City that will give you some nice views of the city.
While I can’t say that the museum was all that impressive, it was fun enough and it was worth the trip to see the only(?) royal castle in North America, some great views of the city, and some interesting murals and artifacts. The day I went, there was also a free ballet performance on an open stage out in front of the castle’s entrance.