Museo Arqueológico Nacional
This museum boasts three large floors filled with Spanish relics, artifacts, and treasures ranging from ancient history to the 19th century. Among the highlights are La Dama de Elche, the bust of a wealthy 5th-century-BC Iberian woman (notice that her headgear vaguely resembles the mantillas and hair combs still associated with traditional Spanish dress); the ancient Visigothic votive crowns discovered in 1859 near Toledo, believed to date back to the 7th century; and the medieval ivory crucifix of Ferdinand and Sancha. There is also a replica of the early cave paintings in Altamira (access to the real thing, in Cantabria Province, is highly restricted).
Consider getting the multimedia guide offering select itineraries to make your visit more manageable.