One free day: go to Montserrat
With a single day, pick Montserrat, because nothing in Barcelona resembles it. Take an R5 train from Plaça Espanya before 09:00, ride the Sant Joan funicular and walk the ridge first, then visit the basilica and the throne after 12:00. Check the mountain forecast the evening before. If it shows cloud at the monastery, swap to Girona, which works in any weather.























