It's all about the beach
You go to San Sebastian for two things, one is the bars and nightlife (and there is plenty as you would expect in Spain) you can eat and drink all night, and then during the day you do the other thing in San Sebastian to do, go to the beach and surf/swim, kayak, sunbathe etc.
Lovely setup for a town but be prepared for crowds and a bit of bustle cause it sure is busy on the beaches!
Insider Tip:Head up to San Sebastian or at least part of the way to watch the sunset, truely awesome!
Also note that the Alsa ticket office is up the road slighty from the bus stop location it is on Av Sancho El Sablo