I bought a couple of FIIO_KB1K external mechanical keyboards.  It's neat little device, that I would recommend after a couple of weeks use.

It has three blank key caps, a rotary/push knob and inbuilt LEDs for each control.  

There is a web app for it that enables customisation of the key functions.  The default buttons are previous, play/pause, next. The knob press to mute or rotate for volume.  However, with the application you can configure up to five different modes.  You configure the knob to select modes (the instructions for this are bit hard to understand or use the web app.

There are a range of control categories available including keystroke emulation (e.g. ctrl-c), strings, mouse and joystick button emulation, multimedia controls and others I don't understand. Because the configuration is in the keyboard itself, you can swap it between devices.  However, I've ended up with one at home and one at work, with different control modes set up.

You can also customise the LEDs, with different colour indicating different modes.  For example, I have soft amber for streaming, purple for music etc.

The device (once configured) supposedly works with Android and Linux.  So, it might be handy as an external control for a Home Assistant kiosk, an Android head unit or ... media control for a tablet.  We have a little 7" display at home, powered by a Pi Zero and I plan to try it with that.