I found this introduction to Stockholm from the Avid Cruiser:
There is of course more to do in Stockholm but as you usually only have a couple of hours I think you'll be exhausted to try to do more than this.
If you're interested in old ships I recommend the Vasa museum with the old warship from 1628. Ok, as said in the video, it was a big failure as it sank during it's maiden voyage (too high, too little ballast and open cannon doors at the lower level while sailing in high wind) but it's still impressive to look at.
Thanks to "The Avid Cruiser" for the video!