Haven't seen this thread earlier. Have fun on your trip, sound like you're going to have a great time
As I am currently residing in Stockholm, when you drop by here (which you will most likely do when coming from Helsinki), I can suggest you should visit:
Gröna lund. A amusement-park close to the sea in the middle of Stockholm city. It is very enjoyable place during summer evenings. A lot of chicks you can hunt here
Stockholm Palace (royal palace), it's visiable over the river from Gröna Lund but only a 15-20 minute there aswell. One of those "things to see" I guess.
Stockholm City Hall, the view from this old building is amazing, and it is the venue of the Nobel Prize banquet and one of Stockholm's major tourist attractions.
And also, probably the most important: The club-life during the summer nights in Stockholm is immense. You should seriosly consider staying one night here just to see what it is like.