During the summer Trädgården is the place to be if your out clubbing. It has a great festival atmosphere. The chance of you getting seperated from your friends is immidiate but you will meet new ones. I have a love-hate relationship with this place, watch out for the massive que and the ill willing pointers.
A huge flee market that occures two times a year, once in spring the other in the fall. Take the subway to Solna centrum and take the 3 min walk to the big sports arena where the fleemarket is held. There is lots of retro and vintage and a good potential of finding bargains.