Over the course of multiple summers during high school I built a treehouse in the back yard using money I made from working in restaurants. The tree house was slightly limited due to my budget but eventually it was built up into a two level glory. The upper level was a viewing deck with railings, hammocks and a zip line across the yard. The lower level was "indoor" with electricity(extension cord), a bed and decorations. Unfortunately, I wasn't able to maintain it because I went to college and Minnesota's winter ended up destroying it.