Hej.
Once I asked the same question from a original Swede. He answered Arlanda international airport. Because it is closest place in Sweden to warm sunny countries.

But if you mean for spending time in the city, we should consider weather you want to spend time sober or drunk. Since it seems that you are a though Finnish guy and you should drink 2 bottles of Vodka. I suggest you Gamlastan and Slussen and Hornstul. But be careful, Alcohol is very expensive in Sweden. I think it is the case in Finland too.
There are many museums and cultural centers in city. Stockholm is one of the greenest capitals of the Europe (I mean in summer) So you can easily find a piece of nature.