Cuicuilco is a lesser known archaeological site in the south of Mexico City.  It is probably the oldest prehispanic settlement site in the Valley of Mexico.  It covers only quite a small area and you probably only need an hour or so to have a pretty good look around.  Metrobus station Villa Olimpica on Avenida Insurgentes Sur is just a short walk away.  It’s worth a look and on the Sunday afternoon that I visited there were only a handful of others there. The surrounding flora is interesting as well. Entrance is free.