It's just not humanly possible to keep that much milk in your system. Be careful, this is full-powered projection vomiting, so make sure they do it outside or near a big toilet. Now go and collect your money. I don't know why it's so funny to watch people vomit, but it is. The only tweak I would add to this little stunt is taking the vic out for a burrito before the challenge.
Rice comes up real easy like.
Here is a prank for anywhere. Just superglue coins to the pavement. You'll need to find a spot that is both heavily trafficked and suitable for gluing - a front sidewalk or driveway works well, though you may need to clean the surface to get the coins to stick. Another simple antic. The funny part is watching people try to pick up the coins.