Great Halloween subway prank:
One large wad of slightly chewed gum. Must be very stretchy. Wait for doors to close. Place wad with two blobs, one on each door with a "hang" in between. This works best at the most crowded station. As the doors open, the gum stretches, and the pushiest "me-first" types get it all over them. Works absolutely best if you exit via another door before someone points the finger at you.
Then you go to the nearest fast food place with a phone and fill the change return with mustard and ketchup. I never did any of this, only heard about it.