Nearly all the colonies have the same basic things - recycler units for air and water, extra shielding and pulse cannons to fend off stray meteors and drifting space debris, solar arrays to harness sunlight, and fusion reactors in case a meteor made it through to smash the fragile mirrors. While they vary a lot in size and shape, those things are always there.
Nearly all of them also use rotation to fake gravity. It's by far the most cost-effective way. Tractor pads use too much energy. Now, there's only a single distance from the center where you have Earth normal - one gravity. There's a slight comfort zone up and down from that. Some prefer to live in the upper layers, where the gravity is less. The outer levels are used mostly for exercise - the extra gravity makes it a more intensive thing. Of course, then there's the dead center of the gigantic bulk of spinning metal.
That's usually the deal, anyway. Some colonies have wide-open outer space there, preferring the complete donut principle. Most don't, since they prefer the added structural strength by cross-barring the center, but they like to stick other things there, like a fusion reactor or other fun stuff.
The colony I grew up on didn't. Instead, there was a small storage room there. Obviously, it wasn't /meant/ for storage; the guys that built the place must have wanted a playroom. Me and the guys found it by pure luck, roaming through the maintenance ducts on the higher levels, finding one shaft leading right up to spin-center. It quickly became the playroom of us street rats, too. See, it was small enough that you could jump from one side to the other without being smashed into a pulp as you hit the spinning wall, and it was small enough that you could jump right into the spin-center of the whole colony.
That was the fun part - trying to find the still point; the point where you just hang there, room and friends all around you. Sure, after flight came fall, and we all got a fair amount of cuts, bumps and bruises - but as long as we avoided falling into any of the four maintenance shafts, it was okay.
It wasn't just a matter of jumping up there, either. Reaching the place was easy enough. /Staying/ there was the tough part. It took a lot of practice. For those of us who never got that far, there was the in-flight acrobatics. That room saw a lot of laughter, before the plague came. They closed a lot of passageways, including the shafts, to limit the spread of it.
Now, I'm on another colony. Maybe the last I'll ever be on. This one too has an empty center, but it's larger. The fall could be deadly, if not done right. I came here to search for that still point again. It's a very good place to think, or not think at all. Zero gravity out in open space is different; there, you're always falling. In here, you're not, if you keep your balance - which is damn hard with nothing to balance /of/. No ground, no walls, no safety line, no thruster guns.
Old man Pestilence will come looking for me soon - he can guess where I am. I think he actually intends to go through with the whole crazy plan - drop a colony and secure Earth in the aftermath. That's why I'm here. I'm trying to figure out what /I/ want to do. I never signed on to be a pawn.
From the still point, all is shown before you. Nothing escapes you; all revolves around you. That's what I need now - to see everything from the center, to find the balance, the path I need to choose. Maybe, if I rest in the still point long enough, I can find a way to share it with everyone else, too.
But meanwhile, the colony keeps on spinning...