This museum is huge. It gives a complete overview of the different cultures and civilisations which lived over the centuries in the area now known as Mexico. Each period in history with a distinct dominant culture has its own department and collection.
Of course there are some things you can't miss : the atlant from Tula is great, the maya sun calendar is top of the bill, the mexica department with a collection of items about Tenochtitlan (the aztec city of Mexico city) with maps, maquettes and drawings is a must before visiting the centre of Mexico city. And all this is situated in a great park, in a modern building with an innovative architecture. Come see this !