I have another long post in me about the DF Metro, but this one is just about interesting old sites you can get to on the Metro.
Templo Mayor. This is the big one, obviously, and you get to it from the Zocalo stop. There's a museum, a fairly large site (even though most of it is under the Cathedral), signs in English, and lots of interesting views. It's a must-see in DF, but if you're only going to one pyramid site, this isn't my favorite. I think it was 59 pesos for adults and teens. I've noticed a lot of sites in Mexico aren't too picky about our teens paying for tickets, even though my 15-year-old is taller than I am, and I'm technically not short.
Tlateloco. You can get to this one from, not surprisingly, the Tlateloco stop (the Garibaldi stop is about as far away). I like this site at least as much as the Templo Mayor, and it's less crowded and free. Do stop here.
El Conde. Take the blue line to the end at Cuatro Caminos. It's an industrial area with lots of buses and a huge station since it's at the end of the line. You can probably take one of those buses into Naucalpan, but I prefer to walk if possible. It's about a 5 km round trip on foot back to the stop. Go north from the stop, then go left on Toluca to the Periferico. Go under the Pereferico toward 1st of May, cross to the right side of May 1st, turn right on Hormona, then left on Toluca (too bad the Pereferico messes up Touca). Turn right on Ozumba and you'll see the site in a few blocks. We had to round someone up to unlock the gate for us even though we visited during the regular hours.
Mixcoac/Los Pinos site. Despite the various names I've seen for this, the San Antonio stop is the best for this one. Exit from the Metro, cross San Antonio at Revolucion, and turn left on Calle 22. The site is at the end of the road. This one isn't listed on the official INAH website and it was locked when we got there, but we could see pretty much everything there was to see from the fence.
Pino Suarez Metro. This one is in the metro stop. When they were digging for the blue line, they discovered these ruins and now you can see them. It's easier to see them if you get off the blue line rather than the pink, but either way, you can see them. Fun.
Cuicuilco. You have to hike through the university and get across Insurgentes, but this circular pyramid is accessible from the Universidad stop at the end of line 3. It's very urban and an older site than most in the area.
Los Reyes La Paz. You can get to this one from the Los Reyes Metro. We didn't go to this one because the area around La Paz is a litle dicey right now and I didn't want to go out with just my son. But it's less than a 3 km roundtrip back to the stop.
Cerro de la Estrella. We ran out of time to see this one because it's a long way out there nearly at the end of the green line. And I'm not sure there is a lot to see here. But it's near the stop of the same name, so it wouldn't be too hard to check out.
Acatitlan and Tenayuca. These are north of the metro, but I'm assuming you could get to them by bus from the Politecnico stop. I love both of these and they're the best pyramids right in the city.
This is everything I managed to find. It's hard to find sites around DF because every search assumes you want Teotihucan, so I'm proud of this list. If anyone ever finds it useful, I'd love to hear about it.