What waste product does yeast produce?
Yeast produces carbon dioxide gas and ethanol alcohol.
Live yeast can be used to inflate a balloon if you give the yeast something to ferment (such as sugar). They then produce carbon dioxide as a waste product that could inflate a balloon. You should not expect it to be buoyant, however, for CO2 is heavy as gases go (considerably heavier than air, for instance). The yeast cannot use salt for much of anything, however.