Tell me if I'm wrong, but if I recall correctly, Donostia is the city with the most bars per person in whole Europe - the Basque just love to spend all of their free time outside drinking Zurrito (really small beer) and eating Pintxos (see below)
Just in front of the Zurriola beach lies the Ondarra Bar. It's THE in-spot for surfers and the cool crowd of Donosti.. it's a bit rustic and familiar with really nice people.
This is the inside of the Bar Kursaal, close to but not inside the Kursaal cubes. Here you can see the decoration for one of the annual Zinemaldias (the local but famous Film Festival)
This used to be Leize Gorria, the red cave, but sadly it closed its doors in 2011. It was the most grungy, alternative, a bit dirty and anarchist place in the whole city with many live concerts.
Le Bukowski is in the quarter Egia. There are many really good live concerts; it's really basque and alternative and no-one minds if you smoke pot inside
Juantxo is the most known place to get a good but cheap bocadillo (a typical Spanish sandwich normally made of baguette). It's always full of people ordering food they usually take outside; at special events as the Jazz Festival or one of the other festivities it's nearly impossible to get in. And you should now some euskera because the waiters tend to ignore the Spanish language
Inside Juantxo: The bar is stuffed with Pintxos. Pintxos is the most typical thing of Donosti (and the whole Basque country, but mostly Donosti). It basically is a peace of bread with some ham oder fish or something on top. The Donostiarra take this pretty seriously: there are many competitions and people get really creative to overtrump each other
This bar is close to the famous Maria Cristina hotel where during the Zinemaldia nearly all of the stars reside. The walls are covered with pictures and autographs of the actors who visited this small café.
Another bar with pintxos. In every bar you can either take a plate and choose some of the cold ones or you ask the waiter for "pintxos calientes" directly from the kitchen