I've been several times but still haven't managed to rent one of the cabins there. Many people know about this park but there never seems to be that many people there when I go. Go and enjoy the trails and just being out in nature
I wander through Camp Long every time I'm in Seattle. It is very peaceful and the air is so clean! There are miles of trails to walk/run. It is possible to take classes/camps, or rent the cabins and stay overnight, but I haven't done that yet.