Hidden down a narrow alleyway of Via Laietana is this glorious example of Catalan art nouveau - a UNESCO World Heritage Site and a prestigious concert hall. I often go out of my way just so I can pass this exquisite building and stare at it for a few minutes. Built from from 1905 to 1908, it was designed by architect Lluís Domènech i Montaner and is covered in floral mosaics, pink tiles and ornate plasterwork, as well as busts of famous musicians and composers. Inside is just as impressive as the outside, but you can only see it on a guided tour or if you buy tickets to see a concert here. There are often famous musicians and flamenco artists that perform here, so I'd definitely recommend going to see it when there's a show on.