When you get off the train at Xochimilco (it's the end of the line), you can just follow the hordes of people walking towards the water. No doubt you will already have several offers from different boat owners to take a trip on their brightly painted wooden vessels. It is definitely worth haggling with them a bit, I think our final offer amounted to about $5 per person for a two hour ride.
I didn't know much of what to expect on this, as reading a description beforehand didn't do it justice. Basically Xochimilco is a series of waterways and gardens on the outskirts of Mexico City. There are hundreds of boldly colored boats for hire to take you on a relaxing ride throughout. Apparently it is a popular weekend outing for the residents of the capital, although we went during the week and it was fairly deserted.
The scenery is tranquil as you float down the canals, and there are many vendor boats to offer you whatever you're craving. I was fascinated as I watched a woman floating on a canoe, making tortillas by hand and cooking them right on her boat. Another man was cooking corn on the cob on his boat. Finally, there were boatfuls of mariachis who would hang on to the side of your boat and serenade you as the two boats floated down the river together. Lots of people haggling for your dollars, too.
I highly recommend taking a trip in Xochimilco if you are in Mexico City. It is a needed respite from the hustle of the city, as well as being a uniquely cultural experience.