Even older iPads have reasonably fast CPUs and better displays than most of what you can buy off the shelf. Old iOS devices are kind of a pain to deal with but Linux on those platforms would be really useful even if only to turn the thing into a calendar or picture frame.

If people come up with a good way to hack the Apple TV, or at least the older ones that still have the USB-C port, that could be an interesting device.

Otherwise useless phones would be cool as battery backed raspberry like devices