I enjoyed a day-trip by public bus from San Sebastian to Hondarribia, a coastal town to the east that has a picturesque fishermen's quarter (lots of restaurants) as well as a separate medieval quarter up on a hill. Hondarribia's very near the airport.
When I visited San Sebastian in June, a couple of art-related sites were closed. Check with the tourist office before trying to see the Chillida Museum (expected to remain closed for the forseeable future) and the Wind Comb sculpture on the promontory to the west of the city. The latter may be accessible by now, but the road was closed for construction when I was there.
In Zaragoza I recommend visiting the Aljaferia Palace. There's an English audio guide available. The old quarter is very attractive. This city tends to be very hot, so be prepared, though you may get lucky with the fall weather pattern arriving before you do. I found no attractive food options out near the combo train/bus station, which is a good distance from the center of the city but well-served by city buses.
Barcelona has enough sights to keep a tourist busy for months. A few of them are so popular that you'll stand in line for an hour or more if you don't buy tickets in advance. Consult a good, recent-edition guidebook or ask here about the sites you want to see.