I have not been to the Guggenheim yet, so I cannot answer that part of your question, but I have been to San Sebastian.

First, you must realize that the Museo Guggenheim Bilbao is not in San Sebastian, but in Bilbao which is about an hour away and there are other things to do and see in Bilbao as well.

However, if you do get a little time in  San Sebastian (Donostia), then head to the oldest part of the city to get a feel for it. There, you can usually find parking near the Plaza de la Constitución where there are also a lot of restaurants. This is a great place to just walk. If you have time, walk along the water's edge on the beaches just past the Real Club Náutico de San Sebastián. There's a pretty park along the promenade and beautiful beaches as well.  We enjoyed visiting the Catedral de San Sebastián which is an easy walk from the old quarter with a nice shopping district in between.

To eat, stay in the old quarter and stop in one of the pinxos bars. There you can try little breads topped with amazing flavor combinations or different flavors of croquetas. It's all delicious and the pinxos bars are generally fairly priced.

If you have extra time and enjoy a walk and a view, you can head to the little hill between the old town and the ocean and walk up to the old castle - Castillo Kalea.

I only had a day there on my visit, but we did a lot of walking and enjoyed the sites and flavors very much.  Enjoy!