San Sebastian alternatives

My girlfriend is coming to meet me in san sebastian (actually flying into bilbao) after i will have already been there for 4 or 5 days (in July). In the event that I won't want to spend a few more days there, can anyone recommend a town within a couple of hours (train or bus) of bilbao we could go to? We're looking for somewhere we can stay at a nice hostal in a chilled out town next to a beach.

Thanks