Parque del Retiro

One of the strongest memories I have from Madrid is definitly Parque del Retiro, an absolutly beautiful city parc in the heart of Madrid.
Retiro with its 1.4 km² (350 acres) is a huge green spot in Madrid, with its artificial lake on which you can rent small boats, different style gardens, and many architectural and decorative beauties.

What makes this parc even more attractive is the number of sunny days in Madrid, allowing you to go there almost as often as you desire. Having a drink and some tapas at one of the small cafes around the parc and enjoy sunny Madrid is simply great.

From late May through early October, every Sunday at midday, the Banda Sinfónica de Madrid gives free concerts from the bandstand in the park near the Calle de Alcalá. Manuel Lillo Torregrosa composed 'Kiosko del Retiro' to this bandstand.
The Park also features an annual Book Fair.

Around the lake, Retiro Pond, many puppet shows perform, and all manner of street performers and fortune tellers. Rowboats can be rented to paddle about the Estanque, and horse-drawn carriages are available.

Opening hours:
Winter: from monday to sunday, from 6h to 22h.
Summer: from monday to sunday, from 6h to midnight.

To go there:
Métro: Retiro (L2), Príncipe de Vergara (L2, L9), Ibiza (L9), Atocha (L1)
Bus: 1, 2, 9, 15, 19, 20, 28, 51, 52, 79, 146 et 202

For more information, check out Wikipedia's page: http://en.wikipedia.org/wiki/Buen_Retiro_ParkLess