San Sebastián is lovely and has a nice beach, but it is crowded in summer and in winter it is more of a day trip. I was there a few weeks ago...it is pretty small but beautiful. If you are willing to move around you can put together a nice trip by also going to, say, Biarritz, Lourdes, Pamplona, Bilbao, etc. I don’t love a lot of driving with kids, though.