One of my favorite day trips while living in Mexico City was my visit to Xochimilco. Xochimilco, while formerly an independent city, is now actually one of the sections of Mexico City and is known (facetiously) as the Venice of Mexico. It’s a fun outing for people that want to escape the city for a nice day of food and drink on the water aboard small boats called trajineras. Basically, you hop on and float around the hundreds of (crowded) canals just enjoying time with your “shipmates.” You pay by the hour for your time on the trajinera (you pay by boat, not by passenger) and there are vendors that float around on other boats offering up everything from roasted corn on the cob to serenades from live mariachi bands.
If you have more than a few days in Mexico City and a great group of friends to go with, I would definitely recommend this outing.