We thoroughly enjoyed our stay in Donostia. The flat was well-appointed, large, clean with a big terrace, close to a supermarket, and about 10 minutes' walk to the sea and the old town. It was extremely quiet at night, yet close to plenty of bars and restaurants.
It's advisable to book any local restaurants, especially at the weekend, since we found them all to be full, with availability only much later at night. Pintxo bars were no problem though, but all pavement seating was full in the evening.