Check out old town, specifically the viking bar down there. Tis amazing... Heres a link:
http://aifur.se/en-GB/aifur-en/
Also, be fully prepared for very strict entry into clubs, it's not uncommon for people to be gathered around the door and the doormen selecting who they want in based on current people inside. It's not a queue system, more of a 'who looks hip and who doesn't' situation. That and most clubs/bars will be empty until after 11pm.
I'd also suggest other locations for a weekend pissup, Dublin is absolutely awesome fun for a bunch of drinks and activities, if you don't mind a day out, there is a shooting range which has clay and target shooting with a falconry right next to it, they serve alcohol (responsibly obviously) so after some shooting, you can grab a couple of beers and handle some awesome falcons and they put on a show. Small group so normally only about 5-10 people on the falconry thing, I'll try and dig a link out from when I went. Also, the bar buskers is a great laugh in Dublin if you don't fancy huge clubs.
Amsterdam obviously...
Barcelona is also a great shout, you can get a 4 bed villa to rent down in Cabrils for pennies and it's a short train ride into Barcelona.
This might not be everyones first thought, but Munich has a great nightlife! Hostels surrounding the park that hosts Oktoberfest are super cheap out of season and also have tonnes of groups there for the same thing so it's easy to make friends and get large groups together for nights out which is what Munich is all about.
If you're looking for more of an adventure style weekend rather than drinking with activities to do, Copenhagen is brilliant, with the Calsberg tour, theme park opposite the main train station, some great places to drink but not many clubs.
Same can be said for Ghent in Belguim, not many clubs, but some awesome sit down bars and pubs with gorgeous views.
Oh! Almost forgot one of my favorite little weekend getaways when going solo or with only a couple of mates, Genoa in Italy has a great nightlife, although not sure on the winter side. There's a dock club where it's half on a dock/peir (private to the club) and half on land which is always busy (in the summer) and in the nooks and crannies of the town it's very hip with modern clubs and bars. And during the day, if you go in the summer, get a 20 min drive east along the coast to just west of Nervi, get ice cream at Gelateria Gaggero, amazing ice cream! There's also a bar on the ocean front a stones throw from the ice cream shop as well.
The worlds your oyster really! PM me if you have any questions on the above or want more, I've been around a bit lol.