You can build a tree house in your back yard as an office or extra room, or guest house or if you really love your kids a play house for them. But I am not so sure you want to waste this kind of awesomeness on kids.
Ok this is amazing! An actual tree stump house! Who wouldn't want this as a secret den at the bottom of the garden! I would make it some kind of outdoor library/studio space. me and zack are finally living the hobbit dream!
For those that don't want a playhouse in their yard why not create a nest for the kids. This way you have a place for children to play and a unique piece of garden art that you get to design and create.