When you're interested in politics or on a rainy day, the swedish parliament house - Riksdag - can be visited. It is open for anyone wishing to attend a debate or public hearing or you can participate in a guided tour, where the swedish political system is explained, you learn about the history of the Riksdag and you're guided around in the building to see some interesting and important places.
The entrance is at Riksgatan 3. During summertime (June to August), there are tours from Monday to Friday, four tours per day in english and five in swedish. From middle of September until June, guided tours are only on Saturdays and Sundays. The visit is free and you don't have to book in advance. Find all the details at the official page of Riksdag.