One of the things that I really loved to do before the advent of the internet was to page through books and photocopy cool images for my Dungeons and Dragons group. When they started adding supporting images to modules I was in love. S1, S4 and C1 were truly evolutionary for me. But the best resource I had besides any image that was included in a module was the magazine "National Geographic". You could find the neatest, most amazing, unbelievable things between the covers of those magazines.
It's true that reality is often stranger than fiction. The old adage "You can't make that up...", nearly always holds true when viewing an odd image out of a "National Geographic". Fast forward to the modern era and toss Google into the mix and Viola! You've got one of the coolest inspirational tools at your disposal. I'm going to mention just two small instances of real-world examples that just beg to be ported into your fantasy campaign: Cenotes and Nazca Lines.
A cenote (Wikipedia link) is basically just a type of naturally occurring hole in the surface of the earth, through which the ground water can be easily accessed. These are for the most part a pretty common occurrence on the Yucatan Peninsula. But they are far from common when you see them in real life. They absolutely scream "Cool access to an underground fairy realm!", or "Portal to the elemental plane of water!". I'm sure that you can think of all sorts of things right? You don't have to view them in real life to be inspired, they're seriously cool man.
Now, the Nazca Lines (Wikipedia link) are NOT naturally occurring, which in fact is one of the things that makes these things so damned neat. Simply stated, these Nazca Lines are a series of "pictures" (otherwise known as geoglyphs) that stood for something to the ancient indigenous peoples of Peru. They're massive, sometimes cryptic, sometimes recognizable images of things that can actually be seen from space. Despite the many rumors that surround these lines, it is widely held that they were in fact quite easily constructed by the technology that was readily available to the people of that time. But who cares? Look at these things...Tell me that doesn't make your mind spin.
You look at a Nazca Line and immediate you're thinking ancient space faring race right? Or some kind of cult that created ley lines pointing to deeply important religious sites. These things of course can be stumbled upon by adventurers...maybe deep in a jungle, or on a high plateau, and they'd be tough to recognize from that angle. What are they? These things that stretch off in impossibly straight lines? Off into the far, dusty distance or the emerald shadows of a jungle, they stretch towards unknown treasures and lost cultures.
Just like in real life.