If someone says they can only go to one city in Spain, I would suggest making it San Sebastian. This picture doesn't do it justice.
1 hour flight on Vueling from Barcelona (a pleasant surprise by the way. So clean!) We stayed at Astoria 7 and bought a bus pass from our hotel reception. You can also pay cash (1.60?) Take dbus to the Mt. Igueldo funicular for the attached picture view.
Definitely reach out to a local for a pintxo tour in Old town. Watch sunset and get a drink at La Perla. Walk over for a bite at Bar Narru (the secreto was yummy). A Calimoxo (sp?) is a drink mix of wine and coke. It's what the "college people" came up with to drink but it's actually pretty good and a local drink! Try it. We got a private tour into France and it was amazing.
We went to Martin Berasategui for dinner. The plates were beautiful, I LOVE food and I'll try anything but I can't say I loved the food at Martin B. You might though!...so I say give it a go for a new experience! There aren't many chances to try a 3 star Michelin.
Really there's so much to enjoy and I know we missed a lot! I hope you can stay at least 3 or 4 days.
Note: If you're taking a regional bus (to Pamplona in our case) the offices are in the building to the left (just North) of the station parking lot. We got frustrated looking for it assuming that the offices are within the station lot.