Madrid is now officially one of the greenest cities in the world, thanks to energetic programs for planting a multitude of trees, flowers, and grasslands organized in the last decade by the Comunidad de Madrid. Some parks -- such as Casa del Campo and the Retiro -- have been around for centuries, but new green zones are springing up annually. Overall, the change from just a few decades ago is remarkable.
Casa de Campo (Metro: Lago or Batán) is the former royal hunting grounds -- miles of parkland lying south of the Royal Palace across the Manzanares River. Until 1931 it was exclusively a hunting ground and leisure area for royalty. You can see the gate through which the kings rode out of the palace grounds, either on horseback or in carriages, on their way to the tree-lined park. A lake in the park is usually filled with rowers. You can have drinks and light refreshments around the water or go swimming in a municipally operated pool. Children will love both the zoo and the Parque de Atracciones. The Casa de Campo can be visited daily from 8am to 9pm.
Parque de Retiro (Metro: Retiro), originally a playground for the Spanish monarchs and their guests, extends over 140 hectares (350 acres). The huge palaces that once stood here were destroyed in the early 19th century; only the former dance hall, the Casón del Buen Retiro (housing the modern works of the Prado), and the building containing the Army Museum remain. The park boasts numerous fountains and statues, plus a large pristine and carp-filled rowing lake, whose borders were modernized and waters drained and replenished and between 2002 and 2003. There are also two exposition centers, the Velázquez and Crystal palaces (built to honor the Philippines in 1887 -- see "Architectural Standouts"), and a lakeside monument, erected in 1922 in honor of King Alfonso XII. In summer, the rose gardens are worth a visit, and you'll find several places for inexpensive snacks and drinks. The park is open daily 24 hours, but it is safest from 7am to about 8:30pm.