When I included a rumor about an Invisible Mountain in my Labyrinth Lord Campaign, it was on a whim. I had no idea what the heck what it was - but it sounded like something interesting and very hard to find. Later, I decided it was a structure in the Ethereal Plane.
Then the players decided finally to go to the Invisible Mountain. Crap. Now I had to get serious about it. I was having some painful DM-block until one night, before falling asleep, my crusty old brain opened up and tossed memories of Simon and the Land of Chalk Drawings at me.
For those of you who don't remember, Simon and the Land of Chalk Drawings was a British cartoon about a boy who would visit an alternate world made of chalk drawing and solve everyone's problems there in five minutes or less. It's a theme that was revisited last decade in the cartoon Chalkzone, but there is something inherently creepy about the Land of Chalk Drawings that Chalkzone never replicated.
Mark Myers re-interpretation of Simon was creepy, but in a different way. "Are you looking at me bum? Bum-looker!" ;)
So anyway, the PCs were looking for the Invisible Mountain, and they ran into the family of a crazy guy who had found it decades before. In his basement, they discovered stacks and stacks of stolen slate roofing tiles, all with strange chalk drawings on them. They didn't think much of it.
When the PCs finally went through all the rigamarole to get to the Invisible Mountain, they entered the Ethereal Plane and found it looked just like the Land of Chalk Drawings. Everything was black except the objects had chalk outlines.
I had no idea how the players would react. The guys were pretty freaked out. They were even less happy when egg-shape headed humanoids with no eyes, noses, or mouths began to beat the holy hell out of them. They were seriously hating on the egg-headed dudes.
My biggest worry was that the Land of Chalk Drawings would be silly enough to deflate the players suspension of disbelief. But it worked out just fine, despite my worries - I think especially because there was a real threat of PC death there.
So go watch some of your favorite old cartoons on Youtube and get inspired! Handled correctly, you can drape strange things (The Land of Chalk Drawings) over old ideas (the Ethereal Plane) and freak your players out. And isn't that what DMing is all about? Freaking out people? Right? ;)