For Adults: The HemLoft
This "egg in a tree" hidden on crown land in the woods of Whistler, Canada is called The HemLoft. Built by Joel Allen and his fiancée Heidi using mostly free materials (including $10,000 worth of scraps scored from Craigslist), The HemLoft was secret until it was revealed to the world in Dwell Magazine. In 2013, Allen announced that he'd be posting the HemLoft's materials as a "disassembled DIY package" for someone else to use in Whistler's free section of Craigslist, making the project come full circle.