Spend the day in Barcelona. The RS tour just barely covers the sites in Barcelona.
some ideas: book a food /tapas tour for Saturday evening.
consider visiting the Palau de la musica Catalana and/or The Recente Sant Pau hospital.
Have you read any of Carlos Zafron's novels= Shadow of the Wind? There's a tour that takes you to sites in Barcelona mentioned in the book. Or DeFalcones' Cathedral of the sea= visit Santa Maria Del Mar.
Or if you are interested in the Spanish civil war? there's a great walking tour for that.
Do you like Miro? There's a Miro museum on the way up to Montjuic.
Check to see if they are dancing the Sardana outside Barcelona Cathedral either Saturday at 6:30 ish or Sunday at around noon
I have been to Barcelona twice now, each time for 4-5 days, and I still find lots of new things to do and see.
I think you are underestimating the time to get to and visit Montserrat and Girona, so visiting these places in a half day would be a whirlwind.