I've created and left a 'timebomb' flash drive out in a public place overnight. In order to avoid it being opened by minors, I put all the content into a passworded zip file first, so the only file you could read was a warning that this was part of a game, that it wasn't for children, and the URL with more info, including the password (which required them to again confirm they were over 18).
A lot more safety checks than most of the posts here, but it was still a lot of fun.
See
https://www.reddit.com/r/timebombchallenge for details of the sorts of things (online and real world) that people have done with this idea.