Danish artist Thomas Dambo has hidden 6 "giants" in different secluded locations around Copenhagen. Although technically "hidden", he encourages the public to find and interact with the sculptures, and each has been built to contain unique interactive features, such as birdhouses and a hollow body that can be entered and explored. 

Dambo is passionate about sustainability and "upcycling", so these sculptures are all made of recycled materials as well as some materials taken from the environment surrounding them. Dambo designed each sculpture, but built them with the help of local volunteers. 

via designboom.com