DIY Treehouse. I think we have plenty of potential spots!
wiki how!! 30 steps to planning/building a tree house!
Fun DIY tree house, by suchity such
diy tree houses | Tree house ideas
Dispatches from the Kellogg Furniture workshop.
Tree House Plans - How to Build a Backyard Tree House - Popular Mechanics
If you don't have a tree to build with.... I would also put a screen and maybe a tarp over the opening for bugs and rain
DIY Tree Swing | Backyard Play Area Projects by DIY Ready at http://diyready.com/easy-backyard-projects/
This design makes the perfect tree fort and lookout spot in the garden.