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