I booked an early morning Teotihuacan Pyramids tour with a private archeologist through viator.com. It ended up being a tour group but was pretty cool. They picked everyone up from their hotels and our group was able to tour the ruins before it got too crowded. The tour lasted most of the day and included a stop at a nearby town for lunch and sampling various shots of tequila.
I didn't really have any plans for my trip. I booked a hotel room about two blocks from the Zócalo and decided I'd figure things out once I got to town. In terms of organized tours, I ended up doing the Teotihuacan Pyramids, a hop-on hop-off Turibus bus tour (
http://www.turibus.com.mx/#3 ) and a couple of free neighborhood walking tours by Estacion Mexico (
http://www.freetour.com/mexico-city ).
After that, I had a pretty good feel for what I wanted to see around the city the rest of week and used the subway to get there. Nevertheless, all of the tours were pretty cheap because 1 USD was equal to around 17 Mexican Pesos.