Xochimilco is to the south of Mexico City, and gives a glimpse at the effects of rushed urbanization over the years. It's a great place for tourists. Together with the historical center of Mexico City it forms a UNESCO World Heritage site.
At the Nativitas (not to be confused with the Metro station) embarcadero, take one of hundreds of boats (trajineras) through the canals which is all that is left of the lake on which Mexico City was built. This activity used to be widely enjoyed by Mexicans and it used to be one of the more authentic tourist experiences available. Nowadays however locals no longer consider it a nice experience because of the decay.
The water quality, environmental health, and size of the canal system has declined due to overuse of groundwater, illegally built homes along the canals, and even attempts to fill or block some of the canals. The boats are colorfully painted and often bear the name of the owner's female child or other relative.