Android

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

Make it a server/put LineageOS on it.

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

Backup

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.

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

Turn it into a pihole.

Baby monitor or dash cam.

I use mine as a desktop webcam with droidcam

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

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

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

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

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.

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.

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.

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.

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

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.

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

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