One of my favourite secrets is in the game Mercenary II Damocles on the Atari ST.

You can find the authors house and within his computer where you could do some weird things to the game universe.

Was fun to find the place in such a large solar system and even more fun in messing up the game :)

Indiana Jones character in the LEGO Star Wars: The Complete Saga.

WOOF WOOF

*Silent Hill 2*

In Age Of Empires 2 when you type "how do you turn this on" and unlock the Shelby Cobra which fires bullets.

Ready Player One.
In adition to the end movie easter egg sequence, the movie itself is a collection of easter eggs and references to music, games, other movies.
But it's not a game, or is it?

Level 0 in toejam and real for Sega genesis. I'm not sure if it was actually called level 0, but the way you get there is by using rocket boots, icarus wins or an inner tube and heading to the southwest corner of level 1. Once you get there you will see an island with a hole in the center. Just drop down and it takes you to "level 0".

Level 0 had a hot tub that you could jump in that had hula ladies in it. The hot tub also restored your health. There was also a lemonade stand that would give you an extra life for drinking you first cup of lemonade. There was a lot of loose cash on the ground that you could pick up too. The best part? To leave level 0 you simply let yourself fall off the map and it would put you at the highest level that you made it to before going to level 0. So if you made it all the way to level 15 and decided to go to level 0, once you fell all the way down and left level 0 it would put you back at level 15.

This is also doable in the toejam and earl remake

My favorite easter egg is the one that pioneered the idea in Atari’s Adventure. You find a singular pixel in one of the castles and then use multiple items on a gate to glitch it enough to pass through and you find Warren Robinnet’s credits. It’s historically significant as a reminder that people didn’t used to be credited for their games, so Robinnet sneaked his in there. I had a phase of loving atari 2600 in the late 2000s and I actually did a school presentation on this easter egg. Of course, then the book Ready Player One came out afterward and not only did everyone know of this easter egg but it made new/complete copies of Atari Adventure exorbitantly expensive. The physical retro game market is a racket.

When I was in school, a friend lent me The Rescue of Princess Blobette for the Gameboy. He couldn't beat it and wanted me to tell him how it ends.
I didn't know the game and just started playing without any expectations. After some time the game got so difficult, that it actually wasn't beatable.
The next day my school friend asked me how far I have come, and I answered: "Sorry man, I got to the invisible castle and cleared a few rooms, but later on it's so damn difficult, because I couldn't remember the layout of the visible castle and constantly died in the traps."
My friend looked at me as if I lost my mind.
What I didin't know at the time, is the fact that you are only supposed to play in the normal visible part of the castle. But there is at least one point where you can leave the level geometry and enter some kind of mirrored castle, that has no textures, but otherwise is identical to the visible part.
I have no clue about programming, but I guess, that this is some programming related structure or something necessary for the game to function. In theory the invisible part might be beatable, if you can memorize the whole castle.
No idea, if anybody else ever discovered this.

Well I got to say Heroes of Might and Magic 3 where all the cheats are taken from quotes of Monty Python and the Holy Grail.

Mine is the continuous Commander Keen references in the Doom series. In Doom II, the devs wanted to express their frustrations for having to work on such a kid friendly game (and maybe throw in a pun or two XD). But by Doom 2016 and Doom Eternal, that frustration is gone, like time has healed the wound, and it's replaced by pure respect for Doomguy and Blazkowicz's predecessor.

The Witcher 2 Assassins of Kings

In the game, a dead Assassin's Creed character can be seen near the Bastille Gate on a bale of hay.

One of my favorite Easter Eggs, well it might be cheating, but there's two, but they're in the same spot.

In World of Warcraft: Warlords of Draenor, there's a small island you can fly to in the southwestern part of Talador, on the island there's a very large broken egg with a beach chair and a communication device in tribute to Robin Williams in the show Mork and Mindy. On the same island a few steps away is another tribute, a lamp that if you click on it of course a blue Genie pops out a saying, "INFINITE COSMIC POWER! ...Itty bitty living space" while shrinking back into the lamp. Two very nice tributes to the man on a itty bitty island, always thought it was one of the greatest Easter eggs.

Hmmm, I like the fact that in Persona 4 you visited Persona 3's school. Can't think of many others, Easter Eggs aren't that memorable to me off the top of my head.

I most likely to not win but my favourite secret is the flamingo exit in hitman 2 maiami mission its very funny and goofy