Once a royal retreat, now Madrid's favourite public park — 125 hectares of formal gardens, monuments and genuine local life.
The centrepiece is the Estanque Grande, a rectangular lake where you can rent a rowing boat — a genuinely popular local pastime, not just a tourist activity. The Palacio de Cristal, a stunning 19th-century glass-and-iron pavilion, hosts contemporary art exhibitions and sits beside its own smaller lake. The Rosaleda (rose garden) and the Ángel Caído statue — reputedly one of very few public monuments anywhere dedicated to a fallen angel — are worth the detour.
On weekends, Retiro fills with joggers, families, street performers near the lake, and impromptu drum circles by the Ángel Caído. It's less a "sight to tick off" and more a place to slow down for an hour or two, which is exactly how most madrileños treat it.
Weekday mornings are quietest; Sunday afternoons are liveliest and most local. Spring, when the Rosaleda is in bloom, is the single best season to visit.
The park has multiple entrances — the one near the Puerta de Alcalá (itself worth a look) and the one closest to the Prado are both convenient if you're combining the park with museum time.