This stuff is just classic.
The basic premise of Improv Everywhere is to take normal situations and transform them into a work of art that breaks up the routine.
A little while ago, a large group of folks went into Grand Central Station and all froze for a period of time, then unfroze and went along with what they were doing. Another group got together a bunch of guys who took their shirts off and entered an Abercrombie and Fitch store and walked around for awhile, which I find hilarious.
In this Improv, a musical breaks out in a mall, starting with a fast food worker. Take a gander;