Free Things to Do in Madridmore than a year ago
Also, most of the top museums have an entrance for free one day per week for several hours. For some, it is on Sunday, for others on Monday or after 18h. Check on their websites for the respective offers.
And last but not least, don't forget that most of the religious temples in the city are also entry-free.
Free Tour of Madrid
Tourist Information Centre, Plaza Mayor
Plaza Cibeles, S/N
El Retiro Park
Plaza de la Independencia, 7
Casa de Campo Park
Paseo de Puerta del Angel
Palacio de Cibeles
Plaza de la Cibeles, 1