The former hospital Sant Pau is Barcelona’s window to art nouveau. The moment you enter the gate, you know you’re inside a place that would take you to bygone times. You’d never even know that in the past, these buildings were used as hospitals. From outside, the pointed spires and well-decorated walls and windows give you a Gothic feel but wait until you see its interior.
I’m not really good at remembering addresses but it’s less than 10 minutes walk from Sagrada de Familia. My google map led me there. If you ask a local who lives around Sagrada, there’s no way he won’t know. If you find a gas station at the corner, and facing it, follow the street on its left lined with cafes. The closest subway/metro is Sant Pau or Dos de Main.