The Museo Nacional de Ciencias Naturales is Spain's national natural history museum, with large collections of fossils, minerals, mounted animals, and scientific specimens. It is a good change of pace from Madrid's art museums, especially for families or anyone interested in geology and palaeontology.
The museum is split between two visitor areas with separate entrances, so keep your ticket after entering the first section. Some displays have English explanations, but many are in Spanish only; the specimens themselves still carry most of the visit.