It's disgusting how many things people can still do without being exposed to advertising.
Loudspeakers: Imagine how many speakers there are in the world, and how often they're not playing anything. All loudspeakers should always play ads if not utilized for anything else.
Edit: I went hiking last weekend, and while out there the silence was deafening. Out in the wild, where it's relatively quiet, we'd only need to place speakers every few hundred meters to efficiently broadcast ads to hikers and such.