The Pyramids are quintessential and beautiful, as is the walk from pyramid to pyramid. It's easy to imagine the scene thousands of years ago when the Pyramids were the center of the town. How bustling it must have been! The downside of your visit is that, on a very hot day in Mexico City (on a day where you may not have acclimated yet to the higher altitude), climbing one of these babies can be daunting. The stairs are steep and tiring and, unlike historic attractions in the US, the pyramids are very hands on and railings off. You could easily get brushed off the side of a pyramid to your doom. Yet, that is the beauty of the situation: the photogenic-ness of the scene without a tangle of safety fences in your pictures, the endless opportunity to get the perfect shot of you in front of the sprawling ruins... free of modern adornments other than some ropes going up the stairs for you to grab. Don't feel obligated to get all the way to the top (although I'm sure it's a fantastic view). We made it halfway and there was a breeze and a sweeping view for us. In addition, mostly everyone is trying to get to the top, so there's not much competition on some of the side platforms — from which there are still very beautiful views to be had.
There is plenty to see at the pyramids: The Pyramid of the Sun, the Pyramid of the Moon, the surrounding ruins, the rock foundations of buildings past. There's also a bazaar that sells souvenirs and hats (because you will DEFINITELY need one due to the direct sun) and no shortage of peddlers approaching you (cue the Jaguar sound maker). You can also visit nearby museums which I hear are quite good or go to a local restaurant (a less touristy one if you wish).
Tips: Wear suncreen, wear comfortable shoes — whatever that might mean for you (since one misstep could be dire) — and avoid this place on Sunday. That's the day all the locals get in for free. Also, beware: Bathrooms are few and far between.