Barcelona plus Montserrat plus Costa Brava plus San Sebastian in 9 days?
 

Can we do Barcelona with a side trip to Montserrat, maybe a couple of days in Costa Brava AND see San Sebastian in 9 days? Is that ridiculous? If it's doable, how do you recommend doing it. If we have to drop something, what do we drop? Thanks!

going by car? train?

San Sebastian by ground transport from Barcelona or the Costa Brava will take a fair part of a day. Vueling/Iberia run a couple of non-stop flights a day from Barcelona into the small airport. Second question: Are you flying home from Barcelona? Getting back there will take another chunk out of your limited time. The most efficient route would be a multi-destination ticket flying into Barcelona and out of San Sebastian or, more likely, Bilbao. That city has more flights but is some distance from San Sebastian.
My preference? Stay in Catalonia. Lots to do in Barcelona. Girona, less than an hour by train, is good for two nights, with an easy hop by train to Figueres to see the Dali museum. Or rent a car in Girona for the Costa Brava beaches.

It depends on your priorities, but I think you would be hard-pressed to make such an itinerary work. In contrast, 9 days would give you a chance of seeing Barcelona, Montserrat, and a bit of the Costa Brava.

Personally, I would defer San Sebastian until you have time to see Bilbao and some other parts of the the north of Spain. JMO.

You might find some useful information in my trip report, which covers all of these places.
http://www.fodors.com/community/euro...-catalunya.cfm

If you are full of energy and just want to mark things off a list, then yes, it's possible, but not practical if you want to enjoy the experience.

Would be perfectly doable, but perhaps change the Costa Brava for the Basque coast? Low cost flights from Barcelona to Bilbao all the time (ten flights pr. day, some 60-70€), takes 1h 10 mins. Bilbao is close to San Sebastián. Direct bus from the Bilbao airport to San Sebastián in 70 mins, leaves every hour (PESA bus).

I come back to beautiful San Sebastián all the time since 1987, the culinary capital of Spain, some of the finest city beaches on the continent, a dramatic history and a great and very distinct culture that people take great pride in: http://www.euskoguide.com/places-bas...stian-tourism/

The Basque coast, immediately around San Sebastián: https://tourism.euskadi.eus/basque-coast/

The Basque Country in general: https://tourism.euskadi.eus/en/