Microsoft used to have them in everything, but I believe, and perhaps somebody can correct me on this if I am wrong, that to contract with national governments, hidden or extraneous functionality had to be removed.

I loved the flight simulator and a doom game in excel. Also a few programs had hidden functionality in the about menus.

I suppose my favorite of all time was a hidden menu in an oscilloscope that caused it to play I believe it was pong or some other simple arcade game.

One that if I ever have the time is I want to reverse the very antiquated Apple II or IIgs Where in the World is Carmen Sandiego game, by chance I hit a key combo and the game went beserk.