One of the cool things about Stockholm: It is one of a handful of cities around the world where you can actually land a hot air balloon in town.
So, a unique way of exploring Stockholm is to fly over it, and there are several companies operating balloon tours:
Scandinavian Balloons was started in 1979 and is the oldest ballooning company in Sweden and probably in Europe.
Upp & Ner arranges ballooning over Stockholm (as well as inflatable boat tours in the archipelago, seal safaris, and guided tours on Segways).
Far & Flyg is one of Sweden’s oldest and most experienced ballooning company, with thousands of ascents under its belt.