We just spent a week in San Sebastián. In fact, we are still here, leaving tomorrow. Just a few observations.
Everybody on this forum and in guidebooks recommends pintxos (tapas). Quite a few places in town display the Tripadvisor sticker. Well, in both our opinions, the experience is overrated. The variety is great, but it is not a meal. The latest experience, in a highly recommended bar: We just had four pintxos each, plus two small beer, paid 30 euros, and left hungry. That happen quite a few times. If you want to eat, just go to practically any restaurant in town. The quality of food is mostly excellent, with very reasonable prices.
We visited small towns around SS, both in Spain and France. We liked France better, more picturesque, more "walkable". Visited St. Jean-de-Luz and Bayonne - beautiful places. We also liked Handarribia, Getaria, Zarautz. We're disappointed with Lekeitio and Ondaroia. Of course, we fell in love with San Sebastián, with its great buildings, night atmosphere, warm beaches. Bilbao's architecture is simply unbelievable, and not only the Guggenheim museum, but many many other buildings.
In SS we rented an apartment, in Gros area. Much better than in the Old Town, where it is extremely noisy all night until early in the morning. And it is only three minutes walk to a beach, five - to Old Town. It also came with a very cheap parking (city parking is very expensive, 20-25 euro a day). I do not know if the forum rules allow posting the contact info, otherwise I would do it.