Spritmuseum - the museum about alcohol - must be one of the nicest museums I've seen so far. Really nice decorations, lots of details and you can learn about the creation of alcoholic beverages by reading the cards on the accessories and even try things. Everything is described in swedish and in english. Also, there is a small garden with many plants and you learn about the good and the bad sides of alcohol. Interviews about drinking-habits of different people for example are worth listening to.
The exhibitions are changing regularly, so be sure to check out what's on when you're there on the website.
You find the museum on the Island of Djurgården, leave at Gröna Lund and walk along the water to the museum. The opening times vary a bit every month. Entrance fee is 130 SEK.
Before or after the visit to the museum, have a beer on the bar outside facing the boats and the water. There is also a restaurant with exclusive food.
Definitely worth a visit in my opinon!