Go to app drawer and open Settings, go to bottom and tap About phone
Repeatedly tap Android Version 3 times: It should open a screen showing your current wallpaper and a red Jelly Bean
(This step is optional) Tap the red Jelly Bean once and it will change to a smiling Jelly Bean android head, and the screen will
show the text
Android <4.x version>
That screen is the Jelly Bean easter egg. But don't close it: it has
to be open to enable the...
Android 4.2 hidden daydream:
Tap and hold down on the red Jelly Bean for about 1 sec.
You'll see an animated screen with lots of colorful Jelly Beans and smiling Jelly Beans, which you can play with, pushing
("flinging") them off the screen.
In that moment the hidden feature will be enabled: that animated
screen is known as BeanFlinger and from now on it will be
available (with black background, though) to be selected as one of
the Daydreams. The Daydreams are an Android 4.2 new feature,
consisting of screensaver apps that can run when your phone or
tablet is docked or charging (more info about Daydream).
Enable developer options (If you enabled them, but you want to revert this process, see Note 3)
Go to app drawer and open Settings
Go to bottom and tap About phone
Tap repeatedly Build number 7 times. Starting at 3rd tap, you'll see a toast saying You are now X steps away from being a developer.
After 7th tap, you should get a toast saying: You are now a developer! (If you keep tapping Build number, the toast will change to: No need, you are already a developer.)
The above steps enable the Developer options screen, disabled by default in 4.2 (more info). From now on, you'll see the Developer options in Settings screen.
Tapping the Android Version in About phone screen 4 times will do
the trick too, showing directly the smiling Jelly Bean.
In Christmas season, along with the jelly beans, you'll may see a
wandering candy cane! It works in both BeanFlinger easter egg (left image)
and BeanFlinger daydream (right image).
If you don't need Developer options you can either turn off the switch in that screen (left image) or clear data from Settings app (Settings > Apps > All > Settings) if what you want is to completely remove the Developer options entry from Settings screen (right image).
All of the above work in Android 4.3 (released July 2013) too.