You could do a lot worse than the bar at the top end of Valkendorfsgarten (if you look on Google Earth, you can make out the green awning which covers the extensive smokers area. Bergen is a university town, and this is a popular haunt with the students. Hope you like loud music. Alternatively (or afterwards) there is a half-decent nightclub virtually underneath. Take plenty of money, buy a troll, and take a trip up the Floibanen (funicular). There is also a good Irish pub on the road leading to the Floibanen, for daytime drinking.