Start the day with a good breakfast at La Mallorquina in Puerta del Sol: you definitely should taste a café con leche with a napolitana de chocolate. Then, enjoy the city center vibes.
Visit the Prado Museum, where you can find a lot of Velazquez and Goya paintings (among many others!) and the Reina Sofia Museum, where the famous Picasso’s Guernica stays. Both the museums are for free in some days and times, check the website and save for one more sangria!
Relax in the Parque del Buen Retiro, and enter in the Palacio de Cristal: sit on one of the armchair, get a book, and disconnect. You will see the park all around you and can’t realize you are in a Capital City
Have some fried almonds with pepper and lemon in the Mercado de San Miguel. No need to say any other thing. Ok, let’s just say all the market is an incredible experience of tastes, smells, sights…go there!
Go shopping in the Mercado del Rastro. It’s a bit out of the usual touristic itineraries but very peculiar, they sell everything and I was especially impressed by book sold by weight, that was a completely new thing for me.