How to repurpose an old iPad and make it useful again
After upgrading an iPad, you may want to find a use for your old one instead of disposing of it. Here are ways you can make your older iPads more useful.

Reuse your old iPads.
When your iPad gets old and you want to get a new one, there are a number of ways you can reuse it to keep it useful. In our interconnected world, information is everywhere, and having a second or even a third iPad can come in very handy.
And, even when an app updates cutting off your iPad that may be stuck on iOS 15 or iOS 9, there's still a way to get older versions of software for it. You may have to get the newer version of an app on your new device first, but Apple keeps older versions around.
With that in mind, here are some suggestions on what to do with that old iPad that isn't your main device anymore.
Use your iPad as a second Mac display
If your iPad isn't too told and supports it, you can use your iPad as a second display for your Mac using Apple's Sidecar software.
You must have both your iPad and Mac signed into the same Apple Account to do so. macOS from macOS Catalina onward supports Sidecar, as does iPadOS 13 or later.
You must have any model iPad Pro, iPad 6th generation or later, iPad mini 5 or later, or an iPad Air 3 or later.
Apple has a tech note on how to use Sidecar.
Use your iPad as a photo frame
Since the Photos app is built-in to iPadOS, you can set up Albums in it, and then use slideshow mode to automatically cycle through the photos.
There are a variety of third-party accessories on Amazon and elsewhere that can help make your iPad into a digital photo frame. Special picture frames, slim batteries, wall mounts, and more are available.
There are also third-party apps such as WunderStation which aggregate weather data from various weather stations around the world.

Third party picture frames.
Use your iPad as a weather station
Using the Apple Weather app as well as third-party apps which show NOAA weather radar, you can turn your iPad into a weather station. You can leave the weather apps running and switch between them, or you can remotely connect your iPad to Internet of Things (IoT) devices such as Arduino or Raspberry Pi.
You can set up a remote IoT weather system outside and then remotely monitor its sensors using your iPad to get accurate weather information. Both IoT platforms support a host of weather-monitoring sensors such as temperature/humidity, air quality (for pollution monitoring), altitude, and more.
There are a number of tutorials online describing how to set up an Arduino or Raspberry Pi to act as a weather station.
Use your iPad in your car
If your iPad has cellular service built-in, you can repurpose your old iPad to act as a map reader in your car. Even if your iPad doesn't have GPS you can still use it for maps if your car is within range of free WiFi, or can connect to some form of mobile hotspot.
Using a dashboard or windshield mount, you can attach your iPad to the interior of your car, so maps, music, and more are available at your fingertips. Just be sure not to get too distracted by maps while driving.
There's no good way to induce the CarPlay interface, unfortunately.
Use your iPad as a dedicated book/magazine reader
You can repurpose your old iPad to act as an e-book or magazine reader. A number of online book services such as Apple Books and Kindle are available.
There are also a lot of other third-party booksellers online such as Goodreads.
Some online outlets offer free public domain books and publications such as books from governments. There is also a wide array of medical research available online such as from NCBI and PubMed.
Beyond magazines in Apple News, many magazine publishers now offer digital subscriptions. You can download your favorite magazines directly to your iPad and read them at your leisure.
Use your iPad as a smart home controller
Smart home devices are growing in popularity and you can repurpose your old iPad as a central controller for all your smart home devices. Devices that work with Apple's HomeKit or Google's Nest or Home devices can be controlled via apps, and they don't always have to be the newest ones.
Google also has an online Home service and corresponding apps you can use to control it. Ring doorbells, Nest thermostats, security cameras and systems, and other devices can be controlled using apps on your iPad.

Apple's Home app.
You can also use smart power strips and outlets to remotely control power to devices using apps.
Smart home assistants such as Amazon Alexa and Google Assistant can be used from your iPad to obtain information or control devices.
Use your iPad as an alarm clock
If you travel a lot, or even at home, you can use your iPad as an alarm clock. Using a small iPad stand in a hotel or on your nightstand, you can prop your iPad up and run any number of digital alarm clock apps to view the time.
Using an alarm clock app you can go full screen to display large numerals showing the time, date, and in some cases even the weather.
Apple's Clock app also has an alarm feature as well as clocks for multiple cities and time zones. There are also apps to track your sleep, that don't require the newest and greatest hardware.
Donate, sell, or recycle your iPad
If you don't need an old iPad you can always donate it to someone who does. Or you can recycle it to reduce e-waste and ensure the materials in the iPad are reused for a good purpose instead of ending up in a landfill.
Apple has an iPad trade-in program in which you can get a credit for a new iPad purchase.
Third-party services such as Gazelle and others buy old iPads.
There are also eco-ATMs at various supermarkets which allow you to drop your old iPad into a machine and get a payment for it in-store.
There are plenty of ways to reuse or recycle your iPad, even if it's really old. Definitely don't just throw it in the trash and let it go to waste.
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Comments
Maybe it's different in the US but where I live Goodreads seems to be a book review site, not a bookseller.
I rather use Apple trade in every 3 years to buy new iPad.
I am planning to buy new iPad Pro 11’ M4 with trade in 2nd gen iPad Pro 11’ which Apple will pay above $300 for it.
Since Apple soldered RAM, SSD for all Apple computers, there is no reason to hold the product more than 3 to 4 years. I rather trade in so I can get credit from Apple.
I just joined a rival book review site on the recommendation of a friend that doesn't want anything to do with Amazon: StoryGraph.
Speaking of Sidecar, I later got a refurb 2018 11" iPad Pro, and use Sidecar to connect it (also via USB) to the same MBP, at the same time as the Yam iPad.
So now I have a compact triple display set up on my small home office desk. The MBP and iPads are on separate stands. The iPad stands I got are a bit low, so the bottom edge of the iPads are a little lower than the bottom edge of the MBP display, but it works. The very high DPI of the iPads helps compensate for their small size.
https://www.apple.com/jp/shop/trade-in
Selling a ballooned battery iPad on Yahoo Auctions Japan is not something I was interested in doing because I am cleaning house now in preparation for a move. There are safety issues when shipping something dangerous like to a BUYER that as well. While some may consider it a waste to recycle it, there was good use out of that device through the years, and it was time to be recycled. It's too old and slow to be refurbished. And once you refurbish and replace the battery, that battery too will eventually die, possibly balloon, and then cost even more money to fix.
I no longer really have a use for iPads because I turn to my 16" M1 Max MBP most of the time. Larger screen and better speakers make for great video watching, and I tend to need full access to my computer when I videoconference with people. I often get frustrated with the limitations of iOS. macOS is truly liberating in comparison. That's not to say iPads are bad. Many people like them. I've just been there. Done that. Maybe one day Apple will make a more appealing new iPad for me again. For 2024 though, the Mac is all our family needs. And on that note I'd like to say we still have a late 2009 27" iMac which is in daily use today. Just another example of how the Mac tends to outlast iOS devices. Not having a battery that balloons and destroys your electronics helps a lot!