Aerial photography. It is absolutely mesmerizing. I’ve wasted untold hours just fiddling around with GoogleEarth.
There was an aerial photography exhibit in Millenium Park in Chicago that was just absolutely amazing; it was all aerial photography of the Chicagoland.
Mechanical pens and pencils. I’ve dissassembled numerous mechanical pens and pencils over the years (and usually can’t put them back together). I’m just fascinated by the noises, the motions, and the parts.
I’m also fascinated by weather phenomena. I love to watch a thunderstorm roll in, the way the sky changes colors as the clouds fill the sky, and then of course the lightning and the thunder. If I lived in the Great Plains I’d probably be one of those amateur storm chasers. I also love to watch snow fall.
I can also sit on a river bank and watch the water flow over the rocks. If I see a leaf or some other object floating in the water I’ll follow it until I lose sight of it.
I’m also fascinated by power lines and power poles. I like it when I can see a line of power poles that goes on forever and disappears into the distance miles away.
Cages and crates with slats. You can put something inside them, but still see that thing. Or if they’re empty, you can see through them, but they’re still kind of solid. Solid but not solid. See-through but not see-through.
I love “before and after” pictures, particularly those in cheesey women’s magazines. I also am fascinated by “agony column” letters and advice in similar publications.
In the pet department, dog and cat paws fascinate me. I love to hold 'em and pet 'em.
I love the schedule on my digital cable. I adore navigating the schedule and pushing a little button to read the movie descriptions, find out what year the Seinfeld re-run I’m watching was first aired; these things can fascinate me for hours.
My list includes:
fire (pyro at heart…)
clouds and storms
jets, especially military ones
and aquariums
Aquariums are a big one for me, enough from me to spend WAY too much money on mine. 55 gallon, 75 gallon, 30 gallon, and 5 gallon, all saltwater. I have another 55 gallon that I am waiting to set up once I get some more $$…
I can spend hours gazing into my 55 gallon reef set-up… Watching the corals sway back and forth with the wave currents… The fish swiming around peacefully (usually) and the crabs clean up the scraps…
Oh my gosh, I love this too! I had a total sun moment going to work a while back, I came running in the office to announce to everyone that the sun is hot.
Between the fish tank staring, relating to animals, and musings on the hotness of the sun, I think the mods are going to come in and close down this thread. After all:
I can’t think of a more obvious en masse admission that everyone posting in this thread smokes a lot of weed. 
In terms of what can hold my attention for a long time, flowing water. It’s my element. I just looooove it.
In terms of what endlessly eludes me, no matter how much I think about it: how my sewing machine twists the two threads together. How does it do that? How does it DO that?
For something that doesn’t perceptively move, I can stand on a beach and watch the horizon of the ocean for a long, long time.
I work in a Aerial Photo lab.
Water in any form, really - rain, snow, rivers, lakes, oceans.
On a day-to-day basis - looking out the car window when someone else is driving. Even if I’m not going anywhere new or interesting, just taking the same old route to work every day, I can stare out a car window for hours without complaint or boredom. Watching other people in their cars, watching houses slip by, looking at complex factories and processing plants in the industrial part of town, staring at rolling farmland…it’s all riveting to me. People who aren’t used to being in a car with me tend to get weirded out, thinking that I’m upset because I’m just staring out the window, but I love it.
I like to throw small light items into spider’s webs and then watch them remove the item and/or fix the web. When I was a smoker, ashes worked well.
I get the same feeling when I watch my kitties’ sides go up and down when they breathe.
The fact that the sun is a star. I like to think about what a blue or red sun would look like in the sky.
Dust moving in the sunlight. When I was a kid, I could sit for hours watching it. Now it’s even more fascinating, because I know about Einstein using Brownian motion to prove the existence of atoms.
The Internet- I think about how much it’s changed our lives in the last 20 years.
There’s at least one person in this thread who doesn’t smoke weed- I’m allergic to pot smoke as well as cigarette smoke.
Duuuuuuuude.* You want to borrow my vaporizer? Or hey, I got some brownies. Where did I put those brownies?
I’m fascinated by the way things wear. Sometimes I zone out inspecting the keys on my keyring to see how the whole wear process is coming. I have hardwood floors and would never think of refinishing them because I love the wear patterns. I love to look at old pocket knives that have been carried and used for a bazillion years.
Ditto on:
Maps
Dragonflies
Storms (the kind with lightning)
Snow falling
Moving water
Cats
Dogs
People making things
Beads and pieces of colored glass
People making pottery on a wheel
Hummingbirds and butterflies
The sky. Every freaking time I go outside I actively look at sky. Its just so damn BIG and so full of fascinating things. I find clouds and stars and the sun and moon all amazing and mezmorizing.
Daytime, I’m always looking for rainbows and other atmospheric optical phenomena such as halos and iridescent clouds. I look at this atmospheric optics site over and over and over and scan the sky just about anytime I’m outside looking for any hint of sun pillars or halos or even just looking for cool shapes in the clouds.
At night, weather permitting I always look for meteors and aurora and or even just familar constellations. I love thinking about the vast distances between the stars (some of which aren’t even really there anymore, that never ceases to amaze me!) and noticing the moon illusion.
Also, bubbles.
Ants. I really can’t pass a group of ants by without observing them. I just think it’s so fascinating that they live in this whole other tiny, social world embedded deep within yet completely oblivious to ours.
The movement of water. When, for whatever reason, water is flowing somewhere, I have to watch. I love how it epitomizes grace, completely unpredictable but always perfect. It’s just fascinating to observe.
My almost-seventeen-year-old took Japanese last year, and this summer she’s been memorizing more kanji by writing them on a dry-erase board. Every now and then I look at her bent over the board with her little (home-made) flash cards and her dry-erase pen and I think, “Sqeeeee! Oh, my god, she’s so cute!” etc.
In other words, it may get more rare, but I don’t think it wears off.