Ebbie: at the World Science Fiction Convention in The Hague (1990) there were parrot shaped, helium filled, bahloons. In the tech crew reday room, a few of these were allowed to fly free.
Initially, their strings were sufficient to counteract their willingness to 'settle' on the ceiling. As the gas slowly escaped, their strings were trimmed allowing them to stay almost at ceiling level. However, if you sat still for a few minutes, one would (almost certainly) come and settle down on your head or shoulder. (Presumably thro' convection currents set up by warm bodies, but heating air less than that already heated at the ceiling.)
It was great to watch the reactions of visitors coming in and sitting down for a drink.

Nigel