One minor fascination I have is to cause a helium balloon to remain motionless at one height, without any contact with the ground or a ceiling. This can almost be done by attaching something of positive mass to the relative negative mass of the helium balloon. I say "almost", because however exact the counter-weight is (which can be finely adjusted by adding or removing very small things things like staples from the cord), the whole thing invariably starts to move up or down as minor fluctuations occur in air movement, air pressure, or both. Therefore, you can only get equilibrium for the briefest of moments. In this picture, I've captured the very moment that the top balloon, full of helium, hangs motionless in suspension as it carries the normal air-filled balloon below - its basically in a state of zero mass. For next stage, see over.