The mountain community and monastery of Montserrat takes some effort to get to, but it’s very much worth it. An hour-long train ride from Plaça d’Espanya in Barcelona is followed up by 5-minute-long breathtaking ascent via a cable car or a funicular. (Note of caution: Please make sure you get correct round-trip tickets for the train portion of the trip; there are no ticket booths at some of the stations near Montserrat, and a €25 penalty to exit the turnstiles back in town if your ticket is of the wrong kind.)
Once in Montserrat, there are several attractions to avail yourselves to. Basilica ♥♥ is beautiful, and those willing can endure a long line to see the Black Virgin, a statue from 12th century. Museu de Montserrat ♥ holds a comparatively small collection of paintings, with many important artists (from Caravaggio and El Greco to Monet and Degas) represented by a single work each.
Another funicular can take you to an even higher point, Sant Joan, for panoramic views of the surrounding area all the way to the Pyrenees and the Balearic Islands. The combination of schedule and line size prevented us from taking advantage of that, but there are plenty of other fantastic views all over the Montserrat.