Gas giant atmospheres
More general: Colonisation of the solar system
APM will probably enable regular interplanetary spaceflight thus the gas giant systems will become rather accessible.
Gas giants do not have no a solid surface.
There are three main possibilities too keep something aloft in the atmosphere of a gas-giant
Simple airfoils for aerodynamic lift and new APM enabled long time stable means for propulsion can be used.
Helium does not work as lifting gas in a hydrogen atmosphere. Since there are significant amounts of helium in all gas giants but Saturn (only about 3%) Pure hydrogen can work as lifting gas for very light structures.
Atmospheric helium contents (falling): Neptune ~19% , Uranus ~15%, Jupiter ~10%, Saturn ~3%
Hot gas: Practicability depends on the feasibility of light and effective thermal isolation.
In bigger depths stiff hollow structures keep afloat at a certain depth. At low pressures (<1000bar) density rises distinctly with pressure. Thus unlike submarines the floating hight is self stabilizing. Ships may swing around equilibrium position (depends on damping drag).
Note that even when highly pressurized a hydrogen atmosphere does not become all that dense (mass per volume). It shouldn't be hard to estimate feasibility today. (Lifting vacuum sponges?)
Keeping things of practical use afloat should pose no problems but super-massive objects (like mountains as often seen in fiction) can not be kept afloat.
One of the easiest accessible (but mostly overlooked) source of energy in gas giants is probably geothermal heat. Simply hang down and or lift up a nanotube-cable with ventilated radiators attached at top and bottom. No digging required. The cable acts as a heat conductor better than the atmosphere itself - a thermal short circuit so to say. A thermoelectric, thermomechanic or thermochemical converter then produce the required form of energy. See: Diamondoid heat pump system
Note that in thermodynamic equilibrium a gas in a gravity field has higher temperature at the bottom. Venus (albeit not a gas giant) is a good example. The heat on the surface is mainly due to adiabatic compression and only secondary due to the CO2 trapping radiation. So to extract free geothermal energy from the a planets atmosphere there needs to be more than this natural heat gradient. which is known to be present on gas giants (TODO: find out how much is known about the quantitative magnitude of this excess heatup - e.g. starting with jupiter where we already had a probe in the atmosphere)
Solar energy is weak so far out in the solar system and only useful for very low power applications. On the other hand there is plenty of space on gas giants so at least on Jupiter solar energy might play some role. For harvesting solar energy very light and thin stretched solar-cell-foil protected from the weather in light and thin (fist sized?) balloons filled with helium depleted hydrogen for lift may be usable. Akin to stratospheric mirror airships and airborne mobile carbon dioxide collectors just with closed carbon cycle.
Winds can be strong on gas giants but whats needed for energy generation is gradients of wind-speed. If the gradients aren't very sharp (which is likely with no ground causing turbulences) very large structures would be necessary to harvest wind energy.
On gas giants fusion power is the obvious choice for high power and long term energy supply Their atmospheres are essentially fuel-pools.
There are no heavy elements in the atmosphere for nuclear fission but nuclear fission powered spacecraft (once in use) could potentially "land" into gas giant atmospheres thereby becoming nuclear fission aeroplanes. (TODO: Look at the hypothetical question: Do all the heavy radioactive elements sink when such a nuclear fission aeroplane sinks and evaporates in the depths of a gas giant? Or would that cause longer term pollution?)
Ursanus provides 2,3 % and Neptune 1,5 ± 0,5 % of methane in their atmospheres. Further there are plenty of light non-metal-hydride ices. Given their sizes this is an ridiculous amount of building material. This would allow covering the whole planet in man made flying objects providing a giant area for habitation completely changing the planets appearance.
Gas giants are nice for APM since the resource molecules come in a nice standard form CH4, NH3, H2O, ...
Due to the gas giants very deep gravity wells there is little motivation to move any material out into space (by whatever non-chemical means). One exception could perhaps be Helium3 (if Helium3 fusion gets adopted) since "fusion product speeds" far exceed exceeds escape velocities of all the non-exotic celestial objects that can be found in our solar system. A competing alternative for energy export is performing fusion on the gas giant's "surfaces" and sending it out with EM waves (e.g. lasers with short wavelength that is not yet ionizing => blue).
Beside Jupiter with its very high gravitational acceleration ~2.5g, high radiation planet-moon systems and asteroid sucking tendency. Gas-giants actually and surprisingly would provide a rather nice place to live since the gravity of Saturn Uranus and Neptune is near 1g and the atmospheres nicely shield radiation.
At Saturn there is actually a pressure temperature range where a human can go outside without a pressure suit only with oxygen supply.
Asteroid impacts might be more probable and frequent than on earth. The high escape velocities make leaving the planet hard and dangerous.
Atmospheric drag hinders fast transport (well the same is true for earth)
the mysterious depths
The abyss can be used as ultimate garbage can. Everything tossed down is sinking melting and evaporating. (pollution due to incomplete crackup possible?) Still doing this excessively wastes energy and energy shouldn't be wasted even when it seems abundant.
When liquid density pressures are arrived (~1000bar ?°C) density rises with pressure a lot more slowly.
Special case Titan
Titan is by far the biggest saturnian moon (as big as Mercury) and the only moon in the solar system with a dense atmosphere. The hydrocarbon lakes contain vast amounts of building material greatly exceeding earths hydrocarbon resources. Related water ice as building material.
Saturns rings are believed to be mostly composed out of water ice. But judging from the moons there there should/may be enough organic material for building strong carbon-allotropic structures not just "ice castles". In astronomic measures the pieces of ring material are very near together. This is nice for both transport and communication. Inter-Saturn-ring communication has much lower light-speed run-times compared to inter-asteroid-belt communication. The bigger pieces of ring material have the size of houses but there is a lot like smaller boulders, rocks, gravel and sand. Doing transport that is space-travel in such an environment seems rather interesting: Is there a way to safely navigate in such an environment keeping relative impact speeds manageable. (TODO: investigate this). There will be many opportunities to use advanced APM technology. One of them is safely catching high speed incoming ice particles without the need for regular maintenance. These systems will be way beyond the currently researched relatively dumb "smart" self healing materials which are not atomically precise in nature. (Note that this is also applicable to space debris on Earths LEO.)