There's a brilliant Swiss idea called Pumipumpe. It's just a set of stickers depicting the kind of stuff people have in their home. The idea is that you stick stickers on your mailbox in the communal hallway of your block of flats, declaring what things in your flat you're willing to share. It's brilliantly simple, solving splendidly with stickers the kind of thing people are always trying to solve with apps.
Stickers are like Minimum Viable Entities. They're just enough to demonstrate that something exists and is real, but they're lightweight and disposable and attachable in all kinds of places.