We have a Moto G7 Power which we turned into an e-reader for my wife. She was constantly chewing through her primary phone battery doing this so I offloaded it to the Moto which has been great. With the 5000mah battery (I know, that’s standard these days), and it’s primary responsibility of displaying text on screen, ti’s made a good stand in.
As other’s have said, Android webcam, there are plenty of apps out there, I use “IP Webcam” by Pavel Khlebovich from the Play store which is no cost and does the job quick and dirty.
If you are into home automation, some people have placed old phones near windows and taken advantage of the light sensors built in (Which regulate the auto screen brightness), and used those to trigger indoor lighting events based on the light level where if it’s darker inside, then lights on or gradually turn lights on accordingly. . Or, use the proximity sensor used to turn screen of when near your ear and create a trigger if something came close to it, IDK what you’d want to monitor being close by, but let your imagination fly! Home Assistant is great for this, their companion app, reveals soooooo many sensors on the Android device which can it can act upon. It’s crazy.
Use older apps on the old phone which aren’t compatible with newer Android OS api’s. For example, Prior to them yanking the app, I used an old Samsung phone to use Rockstar’s Ifruit appa as it wasn’t compatible with newer Android phones, to take care of my dog in GTA V.