If you go to "Settings" / "About phone" and tap "Android verion" several times a jelly bean will pop up. Long press it and it confirms that you're running "Jelly Bean" and then gives you a screen with floating jelly beans. The only objective is to flick the jelly beans off the screen. I guess it's a good time killer.
Similarly if you're running CyanogenMod tapping on "CyanogenMod version" will get you their Easter egg.
Engadget had a post on the Easter egg with a video of it in action.