What easter eggs exist in the King's Quest series?

There's that hole in "King's Quest 2," where you peer in it and get a preview for Space Quest. There are a lot of easter eggs in the AGD Interactive remakes and a Mordack Easter egg in the Infamous Adventures remake of KQ3. I don't know of much else

Look for an invisible sign on a tree somewhere on the western edge of the woods in KQ2 AGI.

Look behind the tapestry next to Gwydion's room in KQ3 AGI.

Keep reading the message in the bottle found in the whale's mouth in KQ4 (AGI and SCI).

There's something on YouTube about being able to make the seamonster appear on the sled jumping screen in KQ5, but I'm not convinced it's an actual easter egg, as it looks more like an unintended glitch. This particular KQ installment takes itself pretty seriously, as Sierra showcased this game as a technical marvel at the time. I think from this point on, Mrs Williams protected her work and kept miscellaneous tom-foolery from getting in.

In KQ2, you can sometimes see the Batmobile coming out from Hagatha's cave.

The aforementioned KQ3 Easter Egg isn't just in the AGI version. It's also in the AGDI version. I don't know if it's in the IA version.

I think the thing in KQ5 only exists in the version that doesn't have voice acting.

And the midi-music theme. It was the first Easter egg I remember from a video game. My older brother played these back in the 80s and he found it and cried out and I ran to see and he showed me. It was so out of the blue and unexpected and such a joy to stumble across. If I remember you just have to leave and re-enter the screen like three times in a row or something? So simple, but not something you'd do very often. I've been hunting for Easter eggs ever since!

When I took the screenshot above I had to keep re-entering quite a few more times than three, more like thirty or so. I think there's some small fixed chance for it to happen every time you enter the screen. When I originally played the game in the 80s I stumbled upon it during normal gameplay, I wasn't trying to trigger it on purpose by going back and forth (back then I had no idea of its existence).

And you're right about things like this (and the Space Quest preview mentioned by op) making a lasting impression back then. Especially that Space Quest preview, it seemed like an entire mini movie hidden inside the game.

I think in the AGDI version of KQ2, the batmobile automatically appears the first time you enter the cave screen if it's the first time you've played that game on that computer. I can't confirm that, but it always seems to be true in my experience.