A group of friends can come up with different ideas for various flashmobs. A flashmob is an unannounced and unexpected public event organized by a group of people. The event (it would perhaps be best described as a performance or a social experiment) is meant to surprise/amuse/astonish or even shock people in public.
An example: There's a busy pedestrian street, a metro passage, etc. where people walk back and forth, flashmob organizers blend with that crowd and appear to be random passengers, they distribute themselves sparsely, not to appear to be related as a group, suddenly, all of them "freeze" in various positions to form living sculptures. After some time they start moving again.
This exact flashmob that I described requires quite a lot of people participating in it, to attract significant attention from unrelated people around, but one can come up with many creative ideas for flashmob performances that would involve only a few people and still make a significant effect.
A flashmob is a known concept, I didn't come up with the initial idea for something like that, but it is not a well-known phenomenon and the ideas for different flashmobs are basically limitless. So people could post their original ideas for different flashmobs in the comment section of this contribution.
PS: If you search youtube for flashmob examples you'll find mostly or only unexpected public musical performances, this is not the original concept of a flashmob (even though it could be a musical performance, reducing it to musical performances only takes the whole point of originality and unexpectedness). Flashmob ideas could range from living sculptures that I mentioned to public pillow fights, to people in bikinis sunbathing on a beach in winter, etc.