Day trip idea from Mexico: go float the canals of Xochimilco on those cute little wooden boats called trajineras.
Why you should go to Xochimilco ?
It's a fun thing to do if you are a group in CMDX. Xochimilco is about an hour away from the city, depending on the traffic, of course. We went there with a few other french friends from EDHEC and we had a great time.
The cost is quite low, especially if you are many: 300 pesos per hour. What's cool about these if that you can bring your own drinks and food and have it on the boat, while the owner is moving the trajineras around with a long wooden stick. You can also rent speakers and play music. The trajineras owner will often make breaks so you can go to the restrooms, which are quite rural and would cost you 15 pesos.
-Warning-
The only warning I would give about this place is that, when we were our our way there, some creepy guy drove next to us in a scooter and followed us for about 20min to tell us that the trajinera port where we were aheading was closed and we should go to another one. That was not true at all. He was wearing a tourist badge to give him credibility but really, he was just employed by a company to drive customers away from its competitors.
Don't listen to strangers, some mexicans can have quite original marketing practises ;)