As a backup phone: You can use your old phone as a backup phone, so you can continue to use your old phone when the main phone breaks down or needs maintenance.
As a children's mobile phone: If you have children, you can use your old mobile phone as a children's mobile phone and download some educational applications and games, so that children can learn and entertain without disturbing you.
As a media center: You can connect your old mobile phone to the TV and turn it into a media center for streaming video and music, or connect it to the home office network and use it as a remote control device.
As a camera: You can use your old mobile phone as a special camera to take high-definition photos and videos and save good memories.
As a smart home controller: you can download smart home applications, turn old mobile phones into smart home controllers, and control smart home devices at home, such as lighting, temperature, security equipment, etc.
As a fitness assistant: You can download some fitness applications, such as running, cycling and fitness tracking applications, and use your old mobile phone as a fitness assistant to record your fitness data and progress.
In a word, although the function of the old mobile phone may not be as good as that of the new one, it can still be fully utilized in various ways.