regarding printing useful things: as said before, jigs and fixtures are a great idea, as they sometimes don't have to be too mechanically demanding. Assembly tools, holders, covers

Mechanically functional parts or structural parts can be made but just have to design them with 3DP in mind to avoid its weaknesses

An use that may surprise you: sheet metal tooling. https://www.youtube.com/watch?v=wsxFXTKaXdI You'd be surprised how well it can work!