We had a very nice time in San Sebastian. The apartment was exactly as advertised. It's extremely well served with Metro and multiple bus's from the end of the road, from where the apartment is situated, to down town. If you have the weather and and are feeling energetic you can walk from the apartment easily into the city centre (about 2.5 km) which we did a couple of times.
If you are using the buses, Beatriz can lend you a "Mugi" bus card which is rechargeable, or you could just buy a "San Sebastian" card in the tourist office on the main Boulevard (there's another tourist office outside the main Bus and train station) which we did as it also gives discounts into many of the tourist attractions in town.
The local shops do have a long siesta time (1:30 - 5PM) and are closed on Sunday , which caught us by surprise, but in the city centre every thing is pretty much open during business hours Mon-Sun.