Behind the Prado Museum in the heart of the ritzy Salamanca district, El Retiro Park — once the private garden of Spanish royalty — welcomes kids of all ages. On weekends, vendors sell street food, clowns perform, musicians jam, and friends gather to wander the 350 verdant acres. To burn off a little steam, find the small lake near the northern entrance and rent a rowboat — an especially cooling activity when the weather turns warm.