Itinerary advice- Barcelona+Costa Brava+, Bilbao, + Lisbon?

Hi, I don't think it is practical to group together Portugal and Spain in the same trip, unless you have a significant amount of time. Since it will be the high summer months, have you considered just focusing on northern Spain aka "Green Spain"? It's quite the contrast compared to the south of the country, and looks more like Ireland mixed in with the Alps. The weather is much more temperate. From medieval villages, to the picturesque wine regions, the emerald waters of Bay of Biscay, the culinary delights of the Basque country, the Celtic culture of Galicia, and finally the towering jagged peaks of the Picos de Europa.

Here are few more reasons why you should consider the north of Spain: https://theculturetrip.com/europe/spain/articles/15-reasons-to-visit-green-spain-at-least-once-in-your-lifetime/

Thank you, Carlos!
This was really helpful. After researching these areas of "Green Spain" and reading more about the area, it seems like a perfect way to spend days after Barcelona and still be able to go to Bilbao. We may just fly out of Madrid. The weather does seem much more temperate in August and "the medieval villages, to the picturesque wine regions, the emerald waters of Bay of Biscay, the culinary delights of the Basque country, the Celtic culture of Galicia, and finally the towering jagged peaks of the Picos de Europa." have us convinced. Thank you for the suggestions.

No problem, happy to help! I feel the north is best experienced with a car, which allows you to make stops along the way. With 14 days, you can do a nice little sampler road trip of Northern Spain, here would be my recommendation:

Barcelona + daytrip (4 nights)

Take the train or fly to

Bilbao (3 nights) - day trip to San Sebastian

Rent car, drive to

Fuente Dé Parador or Potes (2 nights) - explore Picos de Europa National Park

Drive to

Oviedo (2 nights) - explore Asturian coastline

Drive to, drop off car

Madrid (3 nights)

fly out

I'm glad you like Carlos's idea, because I really enjoyed my time in that part of Spain. I just want to mention that the fastest transportation between Bilbao and San Sebastian is by bus, which runs frequently. There's a train, and the scenery is very nice, but it's much slower than the bus. If you have some extra time, I think it's worth going one way by train, but you probably will have more than enough to fill your time without doing that.

Oviedo is an historic city with some very old churches. It's relatively non-touristy. It's not tiny, but I found it uncommonly quiet by Spanish standards. If you prefer lively places, you might want to consider Burgos or Leon instead. I haven't checked to see whether those substitutions would complicate your itinerary because of transportation issues. There's also the weather to consider. Burgos and Leon are far enough from the coast that they're likely to be 7 or 8 degrees (F) hotter than Oviedo, so maybe they aren't such good ideas in the middle of the summer.