I generally come across specific things that prompt me to print a custom solution for my life. I wouldn't be surprised if there literally isn't any truly useful better-3d-printed-than-purchased thing that someone can recommend for you. (But maybe I'm just not imaginative enough :))
Maybe the biggest category of things is replacement parts for stuff around your house. The $7 doohickey might cost you $1+time to print instead.
That said, here are some things I've printed for use around my house, which might give you some ideas for your own life:
- drain spouts with hose connection to route water from my gazebos "gutters" to the ground instead of just splashing down out of a hole
- a cover for the switch on my floor lamp so the light can only be toggled with the wall switch instead of in 2 places
- wall mount for my cable modem/router
- temporary oil cap replacement for my car when I accidentally lost it after changing the oil
- custom tool holders to 5-S my workspace
- large v block clamps for my drill press
- riser for my bench top power supply, to tilt it at a better angle for viewing
- infinite toys for my kids
- replacement part for a model kit that my kid accidentally broke
- handlebar basket custom sized for my kids bike
- replacement latch clip for my car's fuse cover