I'd go to Ostermalms torget. It's square in the centre. A few bars there, ranging from an Irish pub, to a dance-bar to normal hang-out type places. And if you cannot find anything there, you're close to everything else so another set of bars is only a few minutes walk.
I've not been there for a few years, but I remember the bar at the Berns hotel being pretty good, and you could also try the Ice Bar (sister to the Ice Hotel in northern Sweden).