The Jorge R. Acosta Museum is the small archaeological museum attached to the site. It is not a blockbuster, but it helps make sense of what you have just walked through: Toltec ceramics, sculpture fragments, maps of the old city, and excavation context.
Visit after the ruins if you prefer to let the site hit first, or before them if you want a clearer route through the platforms and courtyards. Either way, it is worth the short stop.