Parque El Buen Retiro - once a site only enjoyed by the Spanish Royal Family, it is now a great place to spend an entire day sightseeing through the botanical gardens, kayaking, using the paddle boats, be entertained by jugglers and/or simply sit, relax and enjoy people watching.
It's name stems from what royals used to do here ... simply withdraw from court life and retreat themselves here (whether religiously or not).
It has many lush gardens and a few worth-finding fountains such as the one of the artichoke and the infamous monument to Lucipher - the only one in the world.
For more info.
El Palacio Real
It is no longer a royal residence but it still used to receive dignitaries of foreign countries as well as Heads of State.
Worth visiting: The tour within the palace as well as visiting the Royal Armoury.
Museo del Prado
This is the entrance known as the Velásquez entrance due to the monument that stand before it.
This museum is home to many paintings from re-known Spanish artists such as Goya, Murillo and Velásquez ... as well as foreign artists as El Greco, El Bosco ... amongst others.
Mercado de San Miguel
... a hidden treasure ... you ought to head here to try many seafood delicacies, pastries ... and so much more ...