Many devices require batteries but do not absolutely require to be absolutely portable. Since batteries are expensive and don’t last for very long, it could be a would idea to add a wall power adapter connector to such devices. This would allow you to use the electricity from the mains to power a usually battery powered device.
I did precisely that to my new Air Hog helicopter I got for Christmas (I now, I’m a bit old for toys, but although I did not ask for it, I’m got I got it ). This small helicopter is lots of fun, but batteries (at least the rechargeable alkaline I use) last for about two flights (the helicopter must be charges by the remote in order to be able to fly for about 20 minutes).
Adding a power adapter connection to some device doesn’t require many materials. You only need: a power adapter (duh), its corresponding connector, and some wire.
Once you have all materials, simply locate the ground and V+ nodes on the circuit (usually the black and red wires respectively) and solder the connector to them. Then make a nice hole on the device to make it stick out if necessary and you’re ready to go.
Make sure your power adapter matches the voltage of the device and always verify all voltages with a voltmeter. If you exceed the expected voltage levels for the device you can be pretty sure to fry it.