It's one thing to visit castles, cathedrals, and museums. But a city hall?
Normally, I would have the same reaction. Stockholm's Stadshus, though, is not your average city hall.
For starters, it holds the exclusive and much respected Nobel Prize banquet. The event takes places in a sumptuous room on the second floor of the building, whose gold mosaic-like walls were assembled piece by piece—all 18 million of them—in just a few weeks. Pay attention to what the guide says during the visit; there is much more than meets the eye in this room.
Also, while the building looks like it's about 500 years old, it's not. It's not even 100! It was built to look as though ancient, authentic to the Northern European style of square, red-bricked, austere-looking constructions.
I won't spill all the secrets; there is so much to learn about City Hall that only a guided visit will do it justice. You can either pay the fee or show your Stockholm card to get in free of charge.
Either way, the detour to the somewhat tourist-free Kungsholmen area is well worth it, if only for the spectacular view of Gamla Stan and Södermalm.