Museo Cerralbo

A 19th-century aristocrat's mansion frozen exactly as he left it in 1922: a ballroom lined in gold leaf, armour, tapestries, and a small El Greco tucked among furniture he actually used. Madrid's answer to a life-sized dollhouse of the Belle Époque.

The mirrored ballroom on the ground floor is the highlight, designed to make the modestly sized room feel palatial, and it still works. Free entry applies Thursday evenings and all day Sunday. It's a five-minute walk from Plaza de España, easy to combine with Templo de Debod on the same afternoon.