This is such a cool thing! Imagine, a whole orchestra drifting in to a busy transit station, performing Ravel’s Bolero, and then quietly drifting out again. Take a look!
The Bolero is the perfect kind of piece for this kind of thing, since it starts out with one lone musician, gradually adding instruments as the music builds. It is also recognizable by non-Classical music fans.
The keys to the effectiveness of this performance is that it starts out small and builds, so the “audience” is constantly being surprised and engaged. It is also in a busy, enclosed space with OK acoustics. The piece is readily recognizable to a wide audience. The musicians are in ordinary street dress so they do not draw (much) attention as they drift in to play and then drift back out again. Though I do wonder, how do you bring in timpani inconspicuously? They don’t quite fit in a backpack or luggage trolley!
Of course, I’m inspired to do something similar, but with a flute ensemble. So, what flute ensemble piece would work for this? The players either have to drift in and add to the show, or come together quickly and quietly and start playing with no prep. (In other words you can’t stand there forever trying to find the perfect tuning pitch or the best embouchure placement, just put it together and go!) It has to be clear that you are not busking, that this has the effect of a random event, but not a performance where you are putting on a show and the audience is your captive. You just go and do and leave. One of the best parts of the video is the shot at the end of the lady who is left looking completely befuddled, like “What the hell was that!”
I suppose you could have several small groups performing in different places at once. Instead of flash mobs, maybe you could call them “sparks”! Maybe you could do several small groups leading up to a large group performance. If you did this in a walkable downtown area, choosing some high profile spots to start in, maybe you could draw people with you as you move to the next location, like a Pied Piper effect. Hmmmmm . . . I think I’m on to something here! Better get to work on that. Of course repertoire suggestions are appreciated!