Either add air or remove water from inside the balloon so that it just barely floats. Covering the opening of the balloon, remove it from the cup and drop it in the 2 liter bottle. It should just barely float there too. E. Put the cap tightly on the bottle. F. Try to make the balloon go to the bottom of the bottle and then come back up again without moving the bottle or using magnets.
Air is compressible.
If the air is squished, the balloon will be more dense. Put several 'air-heads' in the bottle so that some just barely float and some sink.