Madrid in a 2-3 days

https://www.ricksteves.com/europe/spain/madrid

You could always do some research on your own. For example, go to TripAdvisor, search on Madrid, then on Things to Do, to see if anything sparks your interest. You could also do a search on “what to see in Madrid.” A bunch of results come up

Here are suggestions on what to do in Madrid from the Madrid Tourism Office.

https://www.esmadrid.com/en/things-to-do-in-madrid?utm_source=portal&utm_medium=menu_Things_To_Do_EN&utm_campaign=enlace_Things_To_Do_EN

The best things to do in Madrid are Museo del Prado, Reina Sophia, and the Royal Palace. If you have time visit the Archaeology museum which is really close to Prado. You MUST get tickets to Prado and the Royal Palace ahead of time online or else wait in a loooong line. I would also get tickets to Reina Sophia ahead of time.

If you want pics without people/crowds get to Reina Sophia when it opens and run to Guernica before 600 people are standing there trying to see it. It's kinda like Mona Lisa that way. The other two places don't allow photographs.

The last time I was at the Reina Sofía in 2017, photographs were not permitted in the Guernica room. Madrid has more to offer than just museums, so ensure you visit some of the non-museum sites.