GeoCaching for offline (semi-online). When I learnt about it, it blew my mind. There's literally trillions of hidden 'stashes' all around the world that people have hidden. In bushes, up poles, behind signs, etc. Every city. Fields, forests, everywhere!
In the stashes you'd find anything from a piece of paper to mini figures or a bullet! You write your name on the paper and date it. Then you hide it and go hunting for more using the Geocaching app.