Great summer activity - this place lets you rent a kayak or a paddle board for a couple of hours and paddle around the little island of Långholmen. The narrow canals on one side are very quaint and filled with little wooden boats, and on the other side you have scenic views of the city hall and the old town. A great way to see the city while getting some exercise, there is also a small beach if you feel like taking a dip (although I recommend just getting in from your board/kayak!).
Not too pricy and super easy; you can pay with a card, usually don't have to prebook, you can leave stuff there and they give you a water proof bag to use.
Alstaviksvägen 3, 117 33 Stockholm, Sweden