Digital photo frame
There are hundreds of Wi-Fi connected digital photo frames to choose from, but why spend money on something you already have. An Android tablet coupled with a dock would be perfect to show a slideshow of your favorite pictures on your tablet. There are dozens of apps to pick from, and you can even use it as a digital photo frame and a bedside alarm clock. Talk about multi-purpose. App for the job Photo Slides
Experimental device
You’ve spent a few hours reading about these cool custom ROMs on the XDA forums, but you are afraid of bricking your shiny new Android device. Maybe you don’t want to lose your warranty. That’s why your old Android smartphone or tablet can be transformed into your development device so that you can explore the wonderful world of Android customization, without having to worry about bricking your device. There are loads of custom ROMs to try, and if you have a particularly popular old smartphone or tablet, then you’ll be well supported by the development community.
Security monitor Camera
If you’ve got an IP camera then you can use your old Android smartphone or tablet as a monitor. Whether you want to keep an eye on the baby or you just want to know who’s at the door, your old Android smartphone or tablet fits the bill perfectly. You can even hook your tablet up on the wall to be used as an LCD monitor for a fixed position in your living room. Audio-only is so 20th century, and those dedicated LCD monitors can be costly, so using your old Android tablet or smartphone would be perfect.
MP3 Player
Are you a heavy music listener? Your old Android smartphone or tablet is perfect for such situations. We are already trying to eke out every last drop of battery life from our Android smartphones, and if you are a particularly heavy music listener, you can waste a good 10% or more in one day of listening to music locally, never mind streaming it.
Luckily, you can use your old Android smartphone or tablet (smartphone would be more preferable in this situation for obvious reasons) as a dedicated MP3 player. This way you can conserve your precious battery life for more important matters. An old Android smartphone without a SIM card is also perfect for your gym sessions, so that you won’t be distracted by any phone calls or texts while you are working out.
E-reader
Phone and tablet screens aren't quite as easy on the eye as e-ink but they're fine for a quick bout of reading on the go. As well as iBooks and Play Books you can use the official Kindle or Kobo apps to sync your reading with a separate e-reader and across multiple devices. You could also use your old phone or tablet exclusively for catching up on your online reading—Instapaper or Pocket are two apps that can help out here.
Twitter ticker
Something of a social media junkie? Apps like TweetBot, Robird andTwitterific can keep your Twitter timeline on constant display and update it with new posts as they come in, assuming there's an available Wi-Fi connection to latch on to. Keep your old smartphone or tablet propped up at the side of your laptop and you can stay on top of incoming tweets without having to keep switching to it in your Web browser.
Radio
While your SIM card may have been switched to your new device, presumably your old one can still use your home Wi-Fi, so why not turn it into a compact radio? Almost every station broadcasts online now—check for official apps from your favorite ones—and there are a pile of online radio apps you can make use of, including TuneIn Radio and iHeartRadio. If your old device comes with a dock, it will work even better.
Kid's toy
If you have children in the house then giving one of them your aging smartphone or tablet is an obvious choice, partly because there's a stack of great educational apps and games around. If you don't have to worry about sharing the device with your offspring then you can tighten up the relevant parental controls and leave them to get on with it, keeping all of your important stuff on your brand new device.