I don't know how much time you have in San Sebastian and Barcelona, but I think Zaragoza is a nice counterpoint. It has some mudejar architecture and feels different from the other two cities. The Aljaferia Palace has been nicely reconstructed and is worth visiting (English audio guide available). It's located roughly midway between the bus/train station and the medieval district. If you're not stopping at the palace, I'd recommend taking one of the city buses running from the station toward the old town. Otherwise, it's a bit of a walk (over 20 minutes, I'd guess). And it can be very hot in Zaragoza.
Note that there's not much in the way of food available in the immediate area of the bus/train station. When I was there in July (maybe low season because of the heat?), the restaurant of the hotel located at the station was closed, with room service available only during the dinner period. There are, of course, lots of food options in the old town.