Treehouses aren't just for kids. In fact, many modern treehouses are built solely to serve the whims of the "big kids".
Very common these days is the B&B in a tree. Many owners of Bed and Breakfasts, in an effort to create a unique and memorable atmosphere, have turned to treehouses. The trend has been confined mostly to Western Europe and the Pacific Northwest but is about to spring forth here on the East Coast. For a sampling, take a gander at:
In the same vein, some have turned to tree top abodes as a unique way of entertaining guests, creating a warm space to enjoy food, drink and company.
The practical minded have realized that a treehouse makes an ideal home office or atelier. Coupling the world's best commute, privacy that can't be beat, and a peaceful vibe that enhances concentration with the ability to write off construction and maintenance costs on your tax return, a treehouse office is a logical choice.
Some want a treehouse for purely hedonistic reasons. They desire a space apart. A place to meditate. A place to chill with friends. A place to play poker. A place to sleep under the stars. A place to reminisce. The child in us never really goes away...