I have such happy memories of doing this as a kid. Emptying out my mom's linen/blanket closet and making a tent city in the living room. Good times... It was awesome to watch the kids come up with the idea on their own. I am always rather surprised in this age of high tech toys at the things that bring the biggest response. When I told them they could use the kitchen chairs, they were happier than Christmas.
Like all good engineers they started to get fancy, they learned which blankets do best as a base, which slip off, they started using the tops to my rubbermaid containers as doors.