We LOVED our stay in Donostia! The beach is gorgeous and the water is warm and pleasant for swimming, unlike a lot of other beaches we visited in northern Spain. We were there (unwittingly) on August 31, for the 200th anniversary of the city's liberation from French, Portuguese, and English forces, and so we got to see a ton of awesome reenactments, traditional clothes, traditional dancing, and traditional music. The city is a mix of tourists, natives, and hippies.
Insider Tip:Keep in mind when you're here that you're not REALLY in Spain - you're in Euskal Herria. Look for Spanish second on descriptions, and English as a distant fourth. Make sure to go to Bar Goiz-Argi and get "gambas," shrimp on a stick with yummy sauce. Tartaletta de txangurro (crab on bread) is also delicious.