I would say: the outside world. Not sure about the effect of this but you could try magazines, (local) newspapers, flyers in your neighbourhood. Although the last two are probably much more effective if your website is focussed on your own region.
Very often I see those QR-code strickers placed on all sort of places, they are really a hype atm in my country, but I don't think they are very effective. I mean which idiot would grab his phone to see what site or message is placed behind the code. Although it might be fun for a few times.
Other (probably uneffective) ways I can think of is wearing a t-shirt with your url on it (or rather paying someone wearing a t-shirt with your url on it), the all time classic of putting your url on a pen, placing stickers with your logo and url in the public area, tagging your url on other peoples walls, creating a crop circle with your url in it and alert the press etc. etc.