Replacement triggers (and other pieces) for old waterguns. Nothing available commercially today comes close to the old supersoakers (the legendary CPS2000 of course, but even the 1000/1200/1500/2500/2700 are pretty rare these days); building your own means a lot of engineering effort and concerns about its safety. So it's well worth being able to fix the old ones, and one of the more common problems is for the trigger (which is plastic, like almost all of the parts) to simply snap at the point where the I-shaped rod joins to the plastic square that you actually move.

This is a pretty niche hobby - I suspect it's a tiny proportion of the 3D printing market - but it's an example of a case where it would be very useful.