cool, probably impractical idea: get a small screen that has a dot matrix display (like one from the old Tamagotchi pets) and program a beating heart animation into it. Then connect it to the battery output (I know I've probably said it wrong. I'm by no means good at any of the technical stuff. anyways, just make it so that it runs on the DMG batteries). And place it in an area where you can see it clearly.
Now, each time you open your DMG (as I assume this would be too big for the MGB and maybe the CGB), it'll become a living, beating 8bit machine! how cool is that? if you have a clear Play It Loud, DMG, maybe you can position it so that you can see it even without opening it.
This will obviously have a drain on the battery, and will of course do nothing else than look cool for some people.