Based on size? Spaceship Earth in EPCOT Center.

On a more personal scale, a lighthouse.

gives OP a hug It’s okay, they mean well.

Inside the fresnel lens of a lighthouse. (The tallest land-based lighthouse in the UK)

Hot air balloon ride wit ma SO,

Safest flight time was in the early AM so we got an amazing sunrise view out of it!

Your mom!

But seriously, a pyramid at Giza. Pretty warm in there after 3500 years in the sun.

The Cologne Cathedral.

It took 500 years to build it.

That is so many generations. Imagine you are just the 4. architect. Your great grandfather has started it, but you did not know this man. Somehow the plans have been passed down to you, but of course there were changes… Somebody after you is going to finish it. You do not know how it’s going to look in the end, because there will be more changes later. If they will be able to finish it at all…

An ancient Buddhist temple in Kyoto. I don’t recall its name without looking it up… But it was much less crowded than most of the other temples and shrines in the area.

I got to climb into a particle accelerator to look at the acceleration coils. Pretty cool.

Hagia Sophia Grand Mosque in Istanbul.

Built in 537 by the late Romans and still standing even though it’s in a relatively active earthquake zone, it’s amazing it’s been around for 1500 years without collapsing. The central dome is so huge and so high, it’s amazing to think how they built it back then without modern machinery or technology.

A nuclear reactor would be at the top of the list, but it wasn’t as fascinating as Dutch windmill when I was 6 years old.

There was a period where I regularly got to go inside Boeing’s Everett factory for work (I didn’t work for Boeing though). For those who don’t know, it’s one of the largest buildings in the world, built in the 60s to manufacture 747s. Now they build all kinds of aircraft there.

“Big” is an understatement. Even “cavernous” falls short. It’s easy for your brain to forget you’re in an indoor space until you look up and see a roof over your head. It’s like a miniature city in there. It’s got its own road network, fire department, cafeterias, and I heard it can even have its own weather.

My route to and from the job site every day took me through alleyways and around sites where workers were actively putting airplanes together. I got to watch an entire fuselage be moved from one side of the factory to the other by the overhead cranes. But my favorite part of the whole place were the underground tunnels that you could use to get around. You could still see old civil defense fallout shelter signs in the stairwells, and even though I wasn’t supposed to take pictures in the facility I did anyway:

There’s not much left of the Mer de Glace glacier, but every year they still carve a giant ice tunnel into it so you can walk into a glacier. Pretty fascinating

Sagrada familia in Barcelona. Genuinely jaw dropping and nothing else has topped it in my travels.

The universe, man.

Back in the late 90s, in the CA East Bay, one of my family’s neighbors was a big shot (director or something, can’t recall) at LLNL’s National Ignition Facility (at the time, it was still called the NOVA laser, I think). My dad got this guy to give us a behind the scenes tour (including clean suit sections) of the complex, including the target chamber where they did inertial confinement fusion experiments (read: shot really fucking powerful lasers with support machinery the size of several contiguous Costcos smashed together at a tiny little gold cylinder with tritium suspended in it), and I got to stick my head in the inspection port.

It was super awesome, and one of the things I credit for making me go down the STEM track in the first place. Also, this was pre-9/11, and in the “peace dividend” era, and I’m fairly certain there’s precisely zero chance a random neighbor kid would be allowed backstage like that in such a sensitive (technically, as well as national security) area these days.

Also, I got to wave at the normie tour group from the other side of the tour glass while in part of the laser hall with our clean suits lol

A chip fab. Highest level clean room at the time. Amazing experience.