There is this group called Improv Everywhere which, "causes scenes of chaos and joy in public places." (their own words) Some of the stuff they do is pretty funny - you can see a lot of it (including descriptions, pictures, and videos) on their website.
One of the funniest things I think they ever did was get a bunch of people (I'm talking like 80 or so) together and have them dress up to look just like Best Buy employees, and then have them all go into a Best Buy in NYC and cause lots of chaos - it sounds ridiculous but really funny too.
