I wish I had taken two days in Mexico City because there are ruins right in the City that I only got to see driving by in a high-speed-cab. There is a Catholic church right down town that is constructed from the stones of a demolished pyramid, but parts of the base of the pyramid still remain right in front of the church.
But yes, be sure to schedule plenty of time to hang out at the Teotihuacan sites. I was one of the only people there that took the time to walk through what appeared to be ancient apartment buildings that are laid out among the pyramids. They had indoor plumbing!!! You can see the water channels from the rerouted river that flowed under the houses. Amazing!
If you're there in Feb be sure to reach out to me. Cheers!