Sant Pau, a UNESCO World Heritage site, was designed by the modernist master Lluís Domènech i Montaner, who also served as the architect for the similarly gorgeous Palau de la Musica Catalana. This is definitely a must-see on your visit to Barcelona! The site also hosts concerts and exhibitions.
The complex is a lovely 20-minute walk from Bailèn 41. Hike to the Sagrada (or take the subway from the Tetuan stop, a half block from Bailèn 41), then veer north on Avenida de Gaudí for four blocks.
After you ooh and ahh your way through the complex and its grounds, pause to refresh at the 1902 Cafè Modernista, located on the grounds and with ample outdoor seating. You can't go wrong.