Just back from Bilbao a couple of weeks ago, second visit after first going in september last year. Took my daughter for a little jaunt as the flights were extraordinarily cheap.. wanted to go again as I love it … superb example of how to regenerate a city in the post-industrial era … just has a great vibe, people nice, and plenty to do for a few days - worth going out to the transporter in Potugalete .. an obvious pull for Teessiders ! It’s a bit more appealing than Port Clarence around there. The food is great in this part of the world. Thr transport in Bilbao is superb, get a Barik card , load a few euros on it and go and have a look about.
San Seb/Donostia is a truly grand place , a city with a beach or three - quite unlike anywhere in UK , again food wonderful.
Do a couple of nights in each and stay as centrally as you can then walk about, stop for food and drink , people watch, enjoy their nice laid back approach. People live in the city centres in big numbers so theres plenty of folk about always. I’d like to explore the region a bit more and may go in the car one time. For now I will be doing a night in Vitoria, Two in San seb and one in Bilbao as the last part of a bigger Spanish trip in June