I love Easter eggs ... the RPG adventure type, that is.
So-called "easter eggs" are the little hidden rooms or interesting areas tucked away in an adventure or dungeon, something that the player characters might discover if they are lucky, clever, or very unfortunate indeed. (I also like the humorous so-called easter eggs tucked away in modules—I've found many in old TSR modules—but that's a topic all its own & one I'll get to another day...) Goodman Games encourages their writers to add eggs into adventures, and some adventures have had some good ones. The GG adventure The Secret of the Smuggler' Cove had what might be the most fiendish I've seen—a cave shrine that may only be reached via a underwater swim. (As a certified SCUBA diver, I can tell you that cave-diving is more dangerous than most non-diver gamers would dream, which just adds to the appeal!)
The best adventure bits for me are the weird ones and the creepy ones. They don't have to be cleverly hidden or impossible to reach, just really interesting and "out of place" in the context of the overall dungeon with being totally jarring. This is not to say I dislike cohesive dungeons—I don't, and good adventure design fairly demands it—but those weird, mysterious touches get me every time. Gary Gygax was a master at this, and finding the little disturbing rooms he salted in his large dungeons was like biting into a big piece of jalapeno in the nacho heap—you knew it when you hit one.
A great example is his venerable Steading of the Hill Giant Chief. The module contains a two-room area on the lower level, just off to the side of the feuding humanoids, that contains a weird little temple and a "vestry" that basically amounts to a blocked passage.
The temple is tiny and bears little connection to the overall place, except to imply that once, long ago, the hill giants (or unknown others) worshipped things better left alone. The place had the standard Gygax descriptions that evoked multiple senses (there were "greasy feeling" columns there!) and it was short & sweet, yet creepy. The fact that a PC could go insane in the room didn't hurt. It was still the days of the 1st Edition, and characters acting greedy in weird temples usually came to bad ends, and quickly...
When I wrote my own The Scaly God (plugga plugga) I made a point of creating my own little weird temple as my easter egg. It's hard to find/reach, and it hopefully plays a bit of easter egg homage to E. Gary Gygax (note the initals—EGG) at the same time. When writing it, I tried to keep my own unique voice in the design as I always do, but all those weird temples over the years, from the awesome Giant series to the Forgotten Temple of Tharizdun, still burned in my brain and it showed. That's a good thing.
In real life, I detest eggs. But in when it comes to adventures, definitely serve me up a couple of easter eggs, weird side up.