This is a bit of a general one, but I have a fondness for very large space structures. A number of my favorite scifi stories take the premise of "exploring a (usually abandoned or tech-regressed) alien megastructure".
Dyson spheres and ringworlds and stellar engines and the like are the obvious candidates, but honestly I like smaller (but still huge) ideas like oneill cylinder colonies just as much, because what I like in them is a function of both size and how close to understood physics they follow, and the latter tend to be easier to make plausible.
There's a certain sense of inspiration that comes of someone describing something almost incomprehensibly huge, explaining that it is designed by intelligent entities, and then justifying it with enough real science as to convince you that something at least somewhat like it really could exist, someday, or maybe already does somewhere out in the vastness of the universe.