Uranus and Neptune

Here's what you do with Uranus and Neptune:
Put a floating refinery in that thick atmosphere. Get the hydrogen, deuterium, and He3, and maybe get some carbon while you're at it, though there are plenty of sources for carbon already. But hey, if you're there anyway...

Use the H, He3, D for fusion. Use the carbon make buckytubes, with an iron atom every now and then so that you can manipulate the cables you'll be making with magnets. The iron can come from iron asteroids or perhaps you find enough of it as you're separating out the lighter stuff. Oh, you'll want to store some of that oxygen and nitrogen, and a bit of other stuff. String the iron-doped buckycables around your ice giant, and use mass-driver-derived technology to spin them up to FASTER than orbital speed. Build a formation of them, say a hundred Km wide, out of, oh I don't know, a thousand buckycables. Have five hundred spinning east to west and five hundred spinning west to east. This way, your cable-driver can sit in one place. It isn't even Neptune-synchronus, but just sitting there, supported by, magnetically levitated above, the cables.

Since it's just sitting there instead of orbiting, it and anything (or anybody) on it experiences weight, so be sure to build it at the best altitude. The best altitude is one above the atmosphere, but close enough to Uranus or Neptune so that the workers on the cable-driver station experience 1 G. I leave it up to those better at math than I am to figure out just how high that is, but if those planets have 1.9 G at the surface, it's going to be a ways.

Now you'll want more than one cable-driver station, to keep the spinning nice and even, so that you have redundancy, and so that you can take one off-line every now and then for inspection and preventative maintenance. In fact, you might as well build a formation of them, too, right around the planet. Then, you might as well connect them all together into a solid ring, stationary, with Earth-nomal gravity, about 160,000 Km (for Uranus) or 155,700 Km (for Neptune) around and 100 Km wide. That's about fifteen and a half million Km^2 for Neptune, and about sixteen million Km^2 Uranus! Actually, it'll be more than that, since the circumference of the ring will be greater than that of the planet.

Now you can either roof the ring over and fill it with air or, if you like open skies, you can build side walls a few hundred Km high, and the planet's gravity will hold the air down, just like on Earth. Either way, everything you need for an Earthly atmosphere is there in the atmosphere of the ice giant, it just needs processing. Use mirrors to concentrate sunlight for illumination and warmth, or use fusion power.

My ideal would be to roof it over, but make the roof ten Km high. Make it a flat roof, and build the high side walls on top of THAT, and fill with air again. Have plumbing, transportation, industry, food production, offices, etc. in the enclosed part, and let the open part vary from fairly "civilized" country estates and parks, to truly wild areas with gazelles, lions, lazurogened mammoths or dinosaurs, whatever. Power your "optimized for civilized" areas with fusion, then warm and light up your "natural" areas with reflected sunlight.

If this ever turns out to be too small, build another ring around the same planet. Repeat as needed. Eventually, you may have a shell around the planet, completely enclosing it. At this point, you no longer need side walls.

And yes, I utterly stole the "supramundane planet" idea. But now I'm NOT a thief, because I'm giving Paul Birch credit.