If you follow the New Nintendo 3DS' family tree back far enough, you'll eventually end up at a primitive black and white handheld device called the Game & Watch that you can now pay homage to with a functional 3DS faceplate from Columbus Circle.
The faceplate is kind of like going out trick or treating on Halloween dressed like your grandfather, but slightly less creepy. It obviously only works with the smaller New 3DS that's currently available almost everywhere except North America, but unfortunately it can't actually be used to play old Game & Watch games. Its tiny buttons are instead used to set a functional digital clock with an alarm and thermometer.
The faceplate will be available from Amazon Japan starting sometime in September for about $US24, so if you've imported a New 3DS, you'll have to import this too. But it would definitely be worth the hassle if it meant you'd never have to open your 3DS (or take out your phone, or check your watch, or look at a clock) again to check the time. [Columbus Circle via Nintendo Tweet via Tiny Cartridge]