An idea that I have but have no desire to act upon that I think would be more effective. Not more so than the banner idea...that is a good one.
The billboard one I think isn't quite as good...and its really pricey. Instead of having one big message in one place I feel like having an interesting compelling message in a ton of places would be good.
Stickers made...flyers...and just put them everywhere in the city. Something people connect with visually but also gets the message across with a website or something.