I booked Barcelona Day Tours to pick us up on debarkation in Barcelona, take us on a tour to Montserrat, and drop us off at our hotel - a 1/2 day tour. They did an excellent job, both during the booking process, and on the tour.
Everything in the booking process was made very clear so that there would be no misunderstandings. The cost depended on whether we wanted a private or shared tour, and they were very flexible about this. If they found more people for our tour, then the shared price was 138 Euro for the two of us. If not, then the private price was 235 Euro total. (I could have also solicited for passengers wanting to share on that other cruise message board, and received a 50% discount, but I was not comfortable with that.)
Our tour guide was Nickolas.
Some photos from Montserrat: