QR code games.

Use a QR Code program, like this one. embed a message, a link, or a picture to it. then leave it where people can find it. When people scan the code with their app enabled phone, they get a hint to where to go or what to do to find the next QR code, which leads to another, and so on.

There are quite a few games using variations of this going on already. It's easy and fun.