If you have a plain old Gameboy or any gadget, water slide decals can help liven them up a bit. I use them to apply graphics to my Game Boys or even model cars.
Step 1: Stuff
Water slide decals can be bought online. Just search for "water slide decal". They might even be in your local craft shops but online is the cheapest place to get them. They are available for inkjets and laser printers.
20 Sheets A4 Laser Water Slide Decal Paper Sheets Transparent Clear
Step 2: Printing
Find your favorite artwork online and then print them off on to the decal sheets. It's best to use a laser printer. Try to use up the whole sheet and not waste it.
Step 3: Cut Artwork
Cut around the artwork as close as possible with a sharp pair of scissors. Be careful.
Step 4: Prepare Device
Make sure your device is clean and it's best to do all your painting ahead of time and allow lots of time to dry. Most of the gameboys I acquire are usually beat up and I give them a bath and then paint them a nice matte white. After that I apply my graphic decals.
Step 5: Applying Decal
As the name implies for water slide decals, you literally soak them in water and slide them off. Get a plate of water and let it sit there for a couple of minutes. It should start to slide and that's when you can place it on your game boy or device.
Step 6: Clear Coat
After the decal is applied and dry. Apply several coats of clear acrylic paint to help protect it. Without a clear coat, it'll wear off from friction.
Step 7: Complete
A super simple project to customize your vanilla Game Boy. Water slides can be used for many other things like mugs, cars, just use your imagination.