I was in San Sebastian just a few weeks ago. Mugaritz is an excellent place and I enjoyed it more than Arzak. I went for a trip to France and visited some wonderful little Basque towns, but i did this because there was a general strike called for that day by the Basque seperatists so not alot was open that day in San Sebastian and I did not regret going. France's border is so close and the town of E'spellete (sp?) is close too...they are famous for thier red dried peppers and the town is very quaint. While you are in San Sebastian go to a cider house just on the outskirts..I had a great time, the food is simple but you get to taste all the cider from the different casks. The one i went too is called Gartziategi. Lot's of good suggestions above from others for pintxos and really exploring the different bars through the old center is part of the charm of San Sebastian. Out by the University there are a few restaurants being opened by younger chefs branching out, one of them is called Agorregl, food was good and a nice looking joint.