I stayed in San Sebastian for a couple of days last year in a place called Pension Bergota which is near the harbour in the old town. Great location and very clean. It is one of the pensions which has a door at street level but the rooms and reception are up some stairs. They didnt serve breakfast but it was not a problem as plenty of cafes close by.
I have done an internet search to try to get you the address but can't find it and the word "bergota" is throwing up some some seedy references so either I have got the spelling wrong or there is a double meaning to the word that is not in my dictionary!!
I found the hotel in a brochure I got from the tourist information office and there were lots to chose from. I suggest you go there and pick up a copy if you can.The pension proprietor was happy to show me the room before I commited myself to taking it. I cant remember the price but it was one of the cheaper options that had a private bathroom.
I also went into the opulent 5 star Hotel Santa Monica to see if they did a special offer for pilgrims but nothing doing and I did feel decidedly out of place with my backpack and crumpled appearance! (Always worth a try!)
I found San Sebastain to be a delightful place, the beach is beautiful -especially after not seeing the sea for weeks. However it was by far the most expensive place I have stayed in Northern Spain. It was quite a shock to my purse after the prices on the camino and even in comparison to the cities like Burgos, Leon and Bilbao. For example, a couple of tapas cost as much, if not more, than a three course meal elsewhere. Anyway, dont let me spoil things for you. I am sure you will love it and even if the fish restaurants at the harbour are expensive the food is great and the views even better.
Good luck and buen camino!