My parents had three willow trees in their yard while I was growing up. One was in the front on the side hill. This was my first tree. It had a double trunk with one side giving dividing even further and then dividing again, all relatively low to the ground. Basically, that side allowed at least two people to sit comfortably in it without requiring too much work. It was my spaceship. Often I would sit in the "gunner" seat (which was higher) while my sister "piloted" our craft. The other side of the tree was a much more difficult climb, but it had a fantastic perch to serve as a lookout post.
The second one was my least favorite as it was near the apple trees and thus more bugs. It also had a low-hanging branch, but it didn't have the multiple perches for more than one person to use. It was, however, possible to hang a tire-swing from it.
While we wouldn't be able to climb the willow in the back until we were older, it quickly became my favorite. It wasn't until about six feet off the ground that this tree split its trunk, but when it did, it made a space like a little tree house. It also had numerous branches parallel to the ground that were perfect for gymnastic moves. Sadly, it is now the last of the willows and all of its large branches have been taken down. All that is left is that trunk which still rises over six feet off the ground.
Today, my mother purchased and had me attach several boards to the willow as well as a couple of hand-holds and a climbing rope. My youngest sons and I climbed to the top as I attached them. It was like visiting an old friend. While it's more of a large stump than a tree, as well as home to an assortment of bug, the boys and I had a fun climbing it again.
I'll just have to check my sons for paper wings before they go to play on it.