One of the most enchanting ideas I've come across in fantasy literature is the old wise man who lives out in the wilderness in possession of many secrets.
I think Tom Bombadil from the Lord of the Rings is the most famous. The Three-Eyed Raven from Song of Ice and Fire is another. But what I'd like to do with mine, if I ever get the chance, is to build a wall around him.
I imagine a circular wall with no entrance. The stones would have to be made of something weird like pink granite, blue granite, or olivine. Olivine might be my favorite since it's the most unlikely and also the color of leaves.
Anyway, like I said, there would be no entrance. The hermit would get in and out of the walled area using a rope ladder. He'd climb up the ladder from the inside, then haul it to the top, and flip it over to the outside when he needed to leave.
Thus, the only way to get it is when the hermit leaves. Then you have to wait for him, which might be days, months, or even years. You'd never know.
The hermit would need healing powers, so a character who is injured might find him and get help. Then the two might form a bond that lasts throughout the story.
On the other hand, maybe the hermit is an old grouch. He refuses help and chases away anyone who comes to him. The characters would then have to figure out a way to placate him and win his favor. Or maybe he's a monster that entraps interlopers.
I'm not hinting that any of this will find its way into my books. In fact, I'm not sure how I could get this motif in at the moment. I doubt I will, which kinda bums me out. But if I could use this idea, I certainly would.
It's just an idea I really like ;)