The Montreal effort.
Buckminster Fuller realized that at a certain level of size, a geodesic dome would hold a considerable tonnage of air.
By simply exhausting a small amount of that air, in effect losing ballast, the whole shebang would become lighter than the volume of air it occupied. Whole cities could be lifted up in a supportive atmosphere.
Of course, just floating around is not a good idea. You have to control where you are going. To that end, I would add some very large and somewhat self-supporting wings to flap, like those of a Manta Ray.
Properly articulated, such a contrivance could hover, or direct its motion even against a mild wind. Certainly it could move along with the wind and go where it was directed.
Buckminster Fuller must have been a blast at dinner parties.