One of the great art museums in Europe — and probably the world — is Madrid’s Prado (Museo del Prado).
My wife and I spent a full day enjoying its fine collection, highlights being the works of Spanish artists like Picasso and Velasquez. The collection is quite extensive, though not as exhausting as say Paris’ Louvre.
Photography is not allowed within the museum, but the grounds and approaches to the museum are very pleasant.