What to do with an old phone

Take the battery out of it. Put on lineage OS. Use it as a microserver, mine some crypto like monero. Because it's CPU-based. You could use it as a remote for other devices. You could use it as a webcam for your house. It can do everything it used to do.

Old lithium batteries become a fire hazard, so I would recommend removing it if you can. Or at least replacing it with the new battery. Good practice at least even if you break it

Leaving an airplane mode, don't use the cellular modem, you're not getting any hardware driver updates. So it's security risk. Don't do anything too sensitive with it. But just for you around home network it's fine

Thanks for you answer !
Why LineageOS ?
- large hardware support portfolio, i think its the single most widely compatible ROM. As long as one crazy maintainer hangs on to your device, you will get lineage updates.
  Just because you forgo hardware driver security updates, doesn't mean you should stop getting android security updates.
Can you run phones without batteries off of AC?
- Depends on the design of the phone. Some will work just fine off of USB power. Some will try to detect a battery and then fail the boot. For the ones that are more troublesome you'll have to Google the instructions on to make a dummy battery load. The important thing is to get the lithium ion battery out of the circuit.

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.

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.

Backup

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

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

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

Superb idea !

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

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.

Turn it into a pihole.

This

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%.

Baby monitor or dash cam.

I planned on using mine for making sort of futuristic terminals around my home. That said, my phones never grow old in my hands. Someone always inherits them before they are due.

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

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.

My old one plus phone is esim compatible and I wouldn't be mad if it broke or was stolen. I've made it my travel phone.

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.

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

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

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.

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