Hi Oona!
You'll LOVE "San Se" in October! What a beautiful city. I spent several nights there during Semana Santa 2003 in
Pensión Donostiarra and we were pleased with it. It's not in "La Parte Vieja" (the old quarter) but is still well located.
What are your requirements? Do you strictly want a hotel or will you also consider hostels and pensions? Are you considering luxury hotels? If so, there are plenty of these in San Sebastián-Donostia. Do you want to be in the Old Quarter, on the beach, room with private/shared bathroom?
Let us know and we'll narrow down our suggestions. Thanks and good luck!
Saludos, MadridMan