Palau de la Música Catalana
This is one the most beautiful music auditoriums in the world. Strangely enough, it’s not very well known (maybe because it’s not in plain sight), but that means you can often visit it without being mobbed by huge crowds. It was designed by another modernist master, Lluis Domenech i Montaner, and finished in 1908. My recommendation is to get a ticket for a concert (any concert), and see (and hear!) the full experience for yourself.