When I was a kid we had a Tree House in the back yard. It was about 8 feet up in a big oak tree. It was a simple creation four walls, a wooden plank floor and a roof covered with leftover shingles. It was nothing fancy, but held a lot of memories in.
Every dream house has a tree house! I love how it is all in one space- the tree house, slide and swings - so only one spot to supervise! And I don't care if I don't even have kids in having this in my backyard