Use it as a remote control
Remote control your TV/music/smart home/... using Chromecast, Unified Remote...
Turn it into a kitchen command center
Dedicated device set up to show recipes, play some videos while cooking, or may be talk while cooking.
Turn it into a dedicated calendar/notification screen
Larger screens are good but seeing your calendar or notifications at a glance may be better.
Turn it into a dynamic photo frame
Auto-display your latest family photos and videos, or may be get the top pictures of the world (like Chromecast does).
Turn it into an alarm clock
You may use your latest device for that, but what about your guest room? Or may be just having a nice stand? Or making sure you've battery and volume set up properly?
Turn it into a smart home sensor
Phones have tons of sensors, network and a battery which makes them decent sensors to build a smart home. They can also be used as smart home control center. Even though they have
Turn it into a dedicate eBook reader
Instead of a library, keep your device there full with books for anyone to use.
Let it power scientific research
You can keep your phone running apps like Folding@home or BOINC to help power computer-intensive scientific research.
Perk - Use it to make money via ads
Perk is an application you can use to stream ads to your phone. It is most effective when running constantly, which means it is best used on an old phone (or a budget phone purch...