There was a proposed method of making a space habitat that I found fascinating, though someone smarter than me would have to address its plausibility. This would have been in a book from Heinlein, Asimov, Niven, or Pournelle, possibly a collection of non-fiction essays on colonizing space.

The idea is that you find a sufficiently large asteroid with high iron content. You hollow out a cylinder in the core and pack it with water ice, and then plug the end. Then you spin the thing on its axis while bathing it with lasers or focused solar light. This heats the surface until it is molten. Eventually the heat penetrates to the water ice core, which boils, inflating the molten iron like a balloon. Once the thing has cooled, you have a hollow space where you can start building your habitat, and a nice robust outer layer, and the whole thing is spinning to simulate gravity.

Again, I don’t know if this falls into the category of “sounds neat but is totally implausible,” or “sounds insane but is actually technically entirely sound,” but it stuck with me as a neat way to build a habitat.