El Retiro is a former royal park which was opened to the public at the end of the 19th century. In this oasis of peace, you can admire the Crystal Palace and the Monument to Alphonso XII, or just stroll around and rent a boat to enjoy a romantic ride on the lake.
Be sure to seek the beautifully shaped trees in the garden - you will know it is them once you see them.
Another interesting sight in the park is The Fallen Angel, which is said to be the only sculpture in the world dedicated to the devil.
Free admission.
Wikipedia https://en.wikipedia.org/wiki/Parque_del_Buen_Retiro,_Madrid
Address Plaza de la Independencia, 7, 28001 Madrid, Spain
Coordinates 40°24'52.344" N -3°40'59.147" E