A visit to Montserrat is a must on any trip to Barcelona (or Catalonia in general).
In the Massif of Montserrat is the Monastery of Montserrat, an important place of pilgrimage and one of the most important tourist places in all of Catalonia. There are many activities that can be done in one day there.
Some of these include visiting the monastery, seeing the Moreneta, taking the funicular railway, enjoying the spectacular views, visiting the Montserrat Museum, listening to the school children sing or hiking. We left the city early in the morning, caught a bus and managed to do some of them!
Although the government has closed access to many of the natural parks due to the high risk of fire, you can still prepare and organize a nice visit to the monastery and a walk nearby. Highly recommended!