I love games with easter eggs. I often put some simple ones in mine like typing "TRIONA" into the title screen on Rockman changes the logo to purple and activates the cheat mode. Sometimes it'll just be to show a hidden credits screen, stuff like that. Reminds me that I've not done anything like that for the last few games I've created /slack
I remember the days of Dungeon Keeper where the game narrator was meant to shout "It's the witching hour" or something similar, if the internal clock on the computer ticked midnight. And, I believe, also a special message if the day was Dec 25th.
It's amazing how many games out there have stuff like this that doesn't get discovered for years. Probably websites out there with loads of examples - time for some constructive (!!) googling.