CAD, 3D printing, laser cutting, and figuring out where things break before they're printed.
Most of my CAD time is in Fusion 360, modeling parts for robots that I then 3D-print or laser-cut at home. Brackets, mounts, custom hubs, the things I can't buy off the shelf.
It's also a thinking tool. Drawing the constraints out in 3D before cutting anything has caught more design mistakes than any amount of staring at a sketch on paper. CAD lies less than your imagination does.
Robots
Most of my CAD time goes into FTC robot parts: brackets, mounts, custom hubs, intake guides. I model the whole assembly in Fusion 360, then 3D-print the plastic parts at home and laser-cut the flat panels. Designing in CAD first catches interference and tolerance issues before I waste filament.
Cycloidal drive arm
I'm building a robot arm around a cycloidal drive of my own design. Cycloidal reducers give a high reduction ratio in a compact package with very low backlash, which is exactly what a small arm joint wants. Most of the parts are 3D-printed; the eccentric input and pin ring are the trickiest to get right. Still in progress.