If your trip to San Sebastián is a relatively short one, then the town does have a well run Boris Bike system called dBizi (the name is a bit of Basque street cred/slang, bizi means life). Cycling is popular in the Basque Lands so there are many private sector bike hire companies in town, with a wide span of bikes from basic to mega spec'd. I've hired a bike there, and the only catch is that it can be hard work, the coastal flat bits aren't that wide!

http://www.donostiamovilidad.com/bicicletas/dbizi/