The jump from Android 4.1 to 4.2 has brought a plethora of changes along with it. In addition to the expanded functionality of notifications, better security, the ability to have multiple profiles one on device, and of course the new keyboard (which you can download now, if you so choose) it just wouldn't be a new version of Android without a couple of cool Easter eggs.
Before we can explain the easter egg its important that you know about a new feature that comes along with an update to Android 4.2. When your phone is charging, either on a dock or plugged into your charger, the screen will display a screensaver like app called a "daydream." Of course since this is Android, you have quite a few options here. Out of the box there are five stock options for you to choose from, but unlocking the fifth takes a little tinkering.
As you may recall in Android 4.1 navigating to Settings --> About Phone and then tapping several times on the 'Android Version' button will bring up a giant smiley jellybean. Hold your finger down on that for a few seconds and your screen will fill with lots of floating, colorful beans that can be flicked in various directions.
Continuing on in that theme, once your device is updated to Android 4.2 going through the same process will unlock a fun little 'game' that will no doubt mesmerize many of us for a short period of time. Once this 'BeanFlinger' daydream is unlocked, all you have to do is navigate to Settings --> Display --> Daydreams in order to activate it.