The plumbing for our swimming pool has a bunch of valves, but we never remember which one is which. 

We originally marked them with magic marker, but that faded and disappeared.

So, rather than take the easy way out and use a label maker, I decided to do this instead :)

I noticed this hole at the top of the valves that I could use for holding new signs
 

So, I fired up Fusion 360 and made this (font changed later)

the sign has a peg to insert in hole in the valve

With a filament change at the right layer, I wound up with these:

The Fusion 360 file is fully parameterized, so you can resize everything. The Text is in its own component, so you just edit the text and export a new STL.