The site museum sits just south of the Pyramid of the Sun, in a low building partly buried into the ground so as not to compete with the pyramid above. The collection is small but excellent: ceramic vessels with the iconic curving stuccoed lids, ground-stone masks, flint blades, and a large scale model of the central city as it would have looked around 500 AD. There is also a glassed-over excavation in the floor showing an intact section of an apartment compound.
Admission is included in the general site ticket. Allow forty-five minutes, especially if it is your first visit — the model is the easiest way to grasp how the whole city was laid out before you go out to walk it. Air-conditioned and a good place to escape the midday sun.