A lot of it is printing things I've found on Thingiverse that other people have created. More recently I've gotten more abilities to use TinkerCAD to build more complex models.
Things I've downloaded and printed: Headphones holder for my new desk, cable hangers for running cat5, Microphone "shock mount", a centrifuge for the kids science project, drywall passthrough grommets, painters pyramids because I forgot to pick some up when I was at the store, Christmas ornaments for us an our family, heart drink coaster for my anniversary.
Things I've designed: Hold-downs for my shop vac, hose adapters for my dust collector, hose adapters for a variety of my tools (many of these hard to just go out and buy), repair parts for my christmas lights to replace broken holders, a tool for more quickly raising/lowering the feet on my workbenches, a customized lathe tool holder for a friend.
The first time I shopped for something on Amazon (the headphone holders), and realized I could print on for a fraction of the cost in 4 hours, I was hooked!