Puerta de Alcalá is, according to Wikipedia, one of the most famous monuments in Madrid with lots of history behind it. It is located next to Parque del Retiro, and I would say that it is the Madrid version of Arc de Triomphe in Paris. A very pretty and photo friendly monument, at every hour of the day.

Walking the streets of the neighbourhood La Latina is defenitely a must when in Madrid. It is a very relaxed and authentic part of Madrid with many tapas restaurants and bars to try. Every Sunday, there is an outside market called El Rastro happening on the streets of La Latina, where you can literally find anything.

When in Spain, you, of course have to try churros. One of the most popular Churro places in Madrid is Chocolateria San Gines which is located close to the metrostation Opera. It is usually packed with people in the mornings but it is worth at least one visit!