treasure hunt

I'm looking to set up a world wide treasure hunt. There are no required special skills. If you live in a capital or major city and are able to place a small sticker somewhere public (legally) or know of a way to place a QR code i provide somewhere where people can find it. I need you. I'll also need your local knowledge to choose a place where you can provide me with a semi cryptic clue to give out. This doesn't need to be hard at all. Just think if someone else from your city had posted the clue, would you know where they're talking about? There may be ongoing hunts. This will need to be done in the next couple months. Just getting numbers and seeing interest. Thanks.