Google Earth flight sim
It's less hidden than most Easter Eggs, given that there's full online documentation on how to use it, but casual users would probably miss Google Earth's flight simulator feature. You access it by pressing Ctrl+Alt+A (Windows), and the user guide (with full instructions) is here. It's not Microsoft Flight Simulator, but it is diverting.
Here are some Easter egg galleries from previous years:
* Looking back at Microsoft Word Easter Eggs
* Looking back at Microsoft Access Easter Eggs
* Looking back at Microsoft Outlook Easter Eggs