In the summer this is a haven in the middle of the city. It is a public garden where I like to go and relax in the grass under a blooming tree. There is also a cafe and a shop here. Before you leave you should buy a bouqee of flowers (you pick yourself in the garden!).
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.
The best late night snack in Stockholm. Together with your last beer, order the french fries with bearnaise at the bar and share it with your friends. Junk food gone deluxe.