I'm going to keep this thread handy. Some nice inspiration.
Hmm. What if my Underdark were to have faerie encounters like this? Interesting. I wonder if I can come up with any ideas that are nearly as good as yours. It's 4am, so maybe the loopiness will assist me.
A man who is a river that tries to flow uphill, but he gets so tired doing so that he drinks himself, and has to rest to regain his body. If the PCs steal his glass cup from him, he'll thank them for stopping him from drinking.
Three shadows, each of which insist it is darker than the others. Of course, they've never seen light, so they have no way of knowing. The party might offer a bit of torchlight, and depending on which shadow they judge to be darkest, they might gain a boon, a curse, or be attacked.
A tiny mischevious sprite that controls a maze of tunnels that have odd echoes to them. The sprite is itself a creature of echo, and it repeats anything anyone says within earshot. You can trick it into doing favors for you by saying something like, "I'm going to show you the way out," which it will repeat and think was its idea. Just never say, "I'm going to kill you."
A pair of stalagmite pillars that sit on either side of a narrow passage. As you walk through, they steal any wood you have on you, because they used to be dryads. They have stats much like xorn. They also collect the skulls of pretty women so they can carve faces onto themselves.