I went to a bar called Zebibo Bar (Plaza Sarriegi 8), which has really good sangria and good music (hip-hop and dance-hall reggae). It has a good vibe, but doesn't pick up until after midnight (as all bars do in San Sebastian).
I also went to Mendaur (St Fermin Calbeton), which has a dance floor and plays house music. Then I went to Amabost Rock Bar, but I don't have many memories of what the place was like, as I was pretty drunk by this point! Hehe...