What to do with an old phone

Make it a server/put LineageOS on it.

Depends on the age of the phone and if it is boot loader locked by a carrier.

I do same. Don't got any accounts logged in either on it and just runs f-droid apps and Aurora store. It's reason I value physical Sims since it makes it easy to swap when I need to without having to sign in to anything, since it's my accountless phone that if I lost has nothing of importance in it.

Become a stalker. Slip into your old girl friends car and use it as a tracker.

It wont last long this way, though
- Attach it to a battery pack.

I use mine as a camera in the garage when I'm working on my car so I don't get my current phone all greasy.

Turn it into a pihole.

This

Backup

Install Pixel Experience, and use it to upload photos to your Google account without consuming your Google Drive data allotment.

Superb idea !

Music player, bedside alarm clock/weather info, time lapse camera

I used to use my old android phone to run an octoprint server.

I use an old Nexus 5 I have as a offline GPS device for cycling.

I use an IP webcam app and run my old S10+ as a CCTV camera inside my garage, viewable in Home Assistant.

Battery powered? Screen on/off?
- The app lets the screen turn off. I have it plugged in to a smart power switch, and have the HA Android app installed.
  This way, I can monitor the phone's battery level with HA and use automation to keep the battery charged between 40% and 80%.

I use mine as a desktop webcam with droidcam

I do the same thing. The camera on my old phone is much better than the one built into my laptop. And even though the phone's battery is pretty much dead, that doesn't matter since I have it hooked up and powered via usb all the time.

Old phone? I'm still running a Mi A1 as my daily. I'm on LineageOS 19.1. There have been rumblings of Lineage 20 coming to it, but the Dev community has largely dropped off.

I'd check if there are any local high-schools or support services that are looking to get phones intot he hands of underprivileged teens/adults. A Mi A3 is likely still better than the low end Samsung garbage, or even an old iPhone (below X).

Better to have it be reused than become a paperweight in a cupboard.

I am hooking up mine to a projector and using it to drive my home theater. Sound is handled by a bluetooth speaker. Is it perfect? Far from it. Is it a better experience for watching movies on TV? Absolutely.

Home Assistant, learn about it. You could turn your phone into a camera viewer, light switch, smart weather provider, detection device with Bluetooth. Etc.

Baby monitor or dash cam.

I usually give mine away, that said by the time I'm done with a phone it's usually pretty used up

You can use it for a Briar mailbox. Briar is a privacy focused messaging with no centralized server.

Instead you can use an old smartphone to act as a server to collect and distribute messages between you and your friends/family.

Next time you upgrade your phone you should try to do a trade in, it bumps some money off of the price and solves the issue of having an old phone laying around. Pretty sure they recycle the parts after that, so win-win.

Most will take the trade in phones, refurbish them, and resale them on a used market platform like backmarket. Anything not worth reselling I think is recycled.
Source: https://youtu.be/WHqxBlFuIXA?si=KC4xxIeI4qPf60QS
When the phone (or whatever) is still usable I always prefer to sell it. You usually get more money and the usable product gets used some more instead of getting destroyed.
- That's essentially what a trade in is

Remote control, Octoprint (if you 3D Print), PiHole, Wall Terminal, Webserver, ...

Mine has broken screen and developer options off. RIP Mi A3.