Long ago, on some early trips to Europe, we had Railpasses or Flexipasses, but for recent trips we haven't had a railpass in years. On our 2013 Spain trip, we took trains several times, but not long enough or enough days to justify a railpass for us. We also rented a car for part of the trip. Cheap air fares - like your $40 Venice flights - could limit the usefulness of a railpass, depending on where you're going and for how long.
We've never used airbnb, but for 2 separate trips to Venice, we reserved places thru Vacanze in Famiglia (http://www.vacanzeinfamiglia.it/?lang=en). The first time was a separate apartment in the owners' home in the Ghetto neighborhood and they served us breakfast, the second time was a ground-floor apartment with full kitchen, and we shopped for our groceries in the Arsenale neighborhood.
In 2013, we flew in to Barcelona, then jumped on the train to Girona (1 hour), and stayed 1 night. We rented a car the next day and drove to the Dali museum in Figueres, then on to the Dali house in Port Lligat. That made for a long day, and we'd reserved a room for the night in nearby Roses. The next day, we drove back to Girona, turned in the rental car, and took the train to Barcelona to finish the trip. That might not be in your plans, but the Dali House was worth the trip!
Haven't personally been to Montserrat, but Rick Steves mentions it in his guidebook and lots of folks on this forum have raved about it as a day trip from Barcelona.
June 21 is a big deal in Europe, with music to celebrate the Summer Solstice - you'll find musicians on almost every corner in cities, giving free concerts.
