1. Turn it into a digital photo frame!
Old phones might be well suited for this job ---simply head over to the app gallery/ Playstore to look for apps such as Fotoo, through which you will be able to configure and display your photo slideshows in a eye-pleasing format.
2. Turn it into a network extender/ wireless access point!
Many modern smartphones have the built-in capability of extending a wireless network by virtue of its own native functionalities or via 3rd party apps.
3. Use it as a second monitor!
With the help of software like Spacedesk, you can create a second virtual monitor as an extended display on your Android mobile device via a wired (USB) or wireless connection.
4. Use it to mine bitcoins while surfing the internet!
Your phone may still be able to support cloud mining apps such as CryptoTab, which is a browser with cloud mining functionalities.
5. Use it to monetize your internet connection!
Your internet bandwidth can be monetized to generate a steady stream of passive income! Apps such as Honeygain, Peer2Profit, EarnApp & TraffMonetizer among many others, can be installed on your old phone to securely 'rent out' the unused portions of internet bandwidth, while keeping your data and security safe. Check out my previous article here!
6. Transform it into a dedicated desk calendar!
Dock your old device on your desk and use it as your personal calendar. Simply use Google's own Calendar app, or search the app store for many other similar apps!
7. Turn it into a security camera!
Consider using your spare phone as a security camera which hooks up to the internet, through which you will be able to access your security footage from anywhere in the world!
8. Use it as a media hub/ TV box!
Hook up your old phone to a USB docking station and turn it into a media center, or even a mini computer!
9. Use it as a wireless network drive!
Convert your existing phone into network-attached storage with the help of apps like Pocketshare, ES File Explorer and many others.
10. Make it a mounted command center for a non-connected car.
Simply mount your phone using a car dock, connect it to a power source, and use it as a GPS device to help you in navigating the roads or even as a music streaming device.