I once read a novel that was set in the clouds of Saturn (or possibly Jupiter, it has been many years!). In best space opera tradition giant warships clashed amide swarms of dogfighting fighter craft. Unlike most such battles, this took place within the atmosphere of the gas giant. The large ships were airships, and the smaller, heavier than air (HTA) aircraft. When a pilot ejected he trusted his life to a balloon rather than a parachute. To descend was to die. Distances were such that some of the HTA aircraft could remain flying for days or even weeks at a time. Some may have been nuclear-powered, and the size of airliners.
The cities themselves were also airborne, resembling gigantic balloons. The basket was the city-proper, resembling a vast plate onto which buildings were erected. Over this was an inflated dome, which contained the breathable atmosphere. Encapsulating this dome was a much larger balloon, containing the helium that kept the city aloft. One interesting feature was that the cities used a helium-oxygen atmosphere. Later in the book the characters visit the Earth and are intrigued at how deep their voices become in a nitrogen-oxygen atmosphere.
Recently I encountered the idea of buoyant habitats once again, but this time in the context of Venus. “DiFate’s Catalogue of Space Hardware” describes the “Cytherea”, from Bob Buckley’s “World of the Clouds” (Analog, March 1980). The Cytherea is a scientific “Skystation” that rides the Venusian jet-stream. The Cytherea was built around a central hub that housed to landing bay for smaller aircraft.
Floating habitats on Venus would float about 50 kilometres above the Venusian surface. At this altitude pressure is equivalent to one Earth atmosphere and temperature is manageable. Above the cloud layer, abundant solar power is available, the large surface area of aerostats providing a large collecting surface. The atmosphere of Venus is mainly carbon dioxide, which is denser than Earth air, so as this blog notes: “This means that the atmosphere of a sizable city could act as both life support and lifting gas. This drastically increases the potential size of a habitat we could build in which to permanently reside. Aside from the multiplication of feasible size, such a city would have other benefits. Due to the pressure of the atmosphere at this altitude, catastrophic decompression of these cities is not at risk. Any tear in the skin of the city would merely cause the air to leak out over the course of thousands of hours. Gasses tend to find homeostasis.”
The official THS-verse does not include any floating cities on Venus. Venus, as described in THS: In The Well, 3e p.68-73, is a somewhat underused setting. It is quite plausible that on the Venus described smaller floating scientific habitats might be found. These would be easier and cheaper to construct and maintain than surface installations. A scenario might be built around a billionaire or collective of investors planning to create the first Venusian cloud city. For the super-rich, Venus offers isolation and privacy, yet with the convenience of shorter travel times to Earth than from Mars.
Note that the possibility of microscopic life existing in the upper cloud layers of Venus has been speculated.