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