There are many cabins around in Scandinavia that stands open for the public free of charge. One of those are Lunsentorpet that you find in nature reserve Norra lunsen.
Watch the video below for more information about the cabin.

The hike started at the parking lot in Heby from where you follow the trail Upplandsleden towards Lunsentorpet. Just after a kilometer you reach another small cabin that is called Fläktanstugan. This cabin is smaller and not intended for staying overnight. But it is a good place to rest and there is a fire stove that can be used if one needs to stay warm. Outside Fläktanstugan there is a watchtower where you can look for birds or other animals in the wetlands. From Fläktanstugan the trail continues towards Lunsentorpet. You walk through some nice old forrest but also some interesting parts of wetlands.

Once reaching the cabin one realise that it is a decent size cabin and can host quite a few people. Apart from the bunk beds that exist there is also the opportunity to sleep on the floor. Outside (and inside) there is a fireplace and firewood is provided. There is also a well for drinking water and a shelter one could use if one feels that the cabin is overcrowded.