The big city square you've seen on TV, in full vibrant color. Of course it's surrounded by fantastic things to see and do. Show up early, get breakfast on the square, then head for the good stuff.
Grandiose as it is, it's also free to come in. The murals by Diego Rivera are the biggest draw, but for architecture and history alone, it's worth spending some time.
The oldest and biggest cathedral in Latin America, this one is worth a good hour or two before lunch. There are of course a good 14 spectacular side-chapels, each crammed with its own gnawing artworks, sculptures, paintings and carvings.
The Gran Hotel Terrace has an excellent terrace buffet with views from about five stories up. Highly competitive, it's in reality one of the best lunch buffets on the market. At about $450 pesos per person, you can eat for less, but for what you're getting, it's not at all an unreasonably priced buffet. Enjoy.
Many guests to the city visit the Anthropology Museum (you saw it yesterday) when what they want is more perspective on Mexico City itself. The Templo Mayor is what they didn't know they were after. While Anthropology can give you a pretty good perspective on the entire country, the Templo museum and site are focused just on what was here.
You get a dramatic and focused insight on where you've been these past two days, and perhaps it will be enough to make you want to come back!