A few favorites:
Plaza Santa Ana.
Oldest restaurant in the world. Restaurante Sobrino de Botin.
The beautiful architecture.
The hustle and bustle of the city.
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Parque del Buen Retiro.
Tapas and more tapas. Paella, jamon iberica, potatas bravas, octopus and croquettes, to name a few. Recommendation: Taberna Alhambra.
And finally, although not photographed, amazing museums such as Museo del Prada and Reina Sofia. Bosch's El Jardin de las Delicias may have changed my life.