Thanks for bringing this up, I'll have to noodle on it a while. Not much is coming to mind off hand, but I'm sure something will.
One thing to consider - an app that can make use of an old cast off, 'out dated' phone, one that no longer has service. A while ago, I realized that these old phones are still potentially very useful - you've got a touch screen, battery, speaker and microphone, camera, wi-fi, etc. Heck, that's a lot for something you probably couldn't get $10 for on the open market!
I was thinking of using a couple I have to monitor various appliances in my home (on/off times of the well pump, sump pump, freezer, etc). But I'm only capable of rather simplistic programming, and none of the simple languages that I could find had much support for reading I/O, so that was a deal breaker for me.
Then I found out about these ~ $9 ESP8266 boards with GPIO, ADC, some flash file space, and wi-fi, and the MicroPython programming language. That fit the bill for me, and I have one up and running now.
It's a variant of Python, which can be useful for scripting stuff on your home computer, so it is a skill that translates to other useful things.
So I probably won't go back to trying this on a phone, but I'll keep thinking about this.
Along these lines - some apps on phones are so much more useful than their hardware counterparts - kitchen timers come to mind. But getting the old phone out, starting it, making sure it's charged - by that time it isn't worth it. But you got me thinking - maybe build an old phone like that into a nice wooden charger base for our phones for the kitchen, and it would always be available and charged to use for those apps, or as a music player, anything that needs nothing more than wifi. I could put some decent speakers in the charger base. Hmmmm.....