Nick's pranks got me thinking about this group called Improv Everywhere
that kind of took the flash mob idea and turned it into elaborate hoaxes. On the surface most of them are very feel good but some of them are pretty dark when you think about them. Here's
one I like where a couple of guys called The Dollar Dudes
were giving away dollars on a NYC subway and they wound up getting challenged by Los Hombres
who show up and start giving away 20 peso bills.
They've done a TON of pranks / hoaxes / performances (whatever you want to call them) and while it takes a bit of investment on your part to check them out a lot of them will make you smile.
Another favorite of mine is The High Five Escalator