Underrated places in Madrid (my opinion as a local)
Templo de Debod
The temple of Debod is an actual ancient Egyptian temple that was rebuilt in the centre of the Spanish capital. It was donated by the Egyptian state in 1968 as a way to thank the Spanish government for its help during the construction of the Aswan Dam, a project that could have severely damaged many of its archaeological sites.
The temple was originally dedicated to the goddess Isis and nowadays is located in Parque del Oeste, a park near the Royal Palace. There is no entrance fee, the views of the city and the palace are incredible and it is the best place to see the sunset. Moreover, it is also a nice area to get away from the heat in the summer and perhaps sit down and have a picnic. There are usually people playing music and really good vibes all around. Definitely my favourite place in Madrid.
Museo Cerralbo
Just around the corner from Debod there is a 19th century building that used to be the home of the Marquis of Cerralbo and nowadays houses his art and antiquities collections.
It is free for everyone on Thursdays from 5pm to 8pm. There is a nice garden with statues and beautiful rooms from the Neo-Baroque and Rococo eras. Less popular than other museums in Madrid (El Prado, Reina Sofia) but not less interesting, especially if you like to see historical objects rather than paintings. Nice place if you want to avoid the sun and the crowds for a while without having to stop exploring altogether.