What to see while they still exist?

I'd say Venice sooner rather than later

And Bosnia & Herzegovina before the start of war. Maybe Georgia before war hits there too. Oh, and Moldova before war hits them any worse.

Oh, you meant things. American muscle cars and and serene vistas not decimated with wind generators ... and whales and the Ohrid Trout and Danube Salmon.

Venice was also my first thought. It simply is sinking into the sea. About half of the population has left. And Miami and parts of keys may disappear.

Agree, Venice.

Cinque Terra is already ruined.

California Giant Sequoias
Arches in AZ
Bisti Bandlands in NM

Antartica

Taiwan?

Some of those low-lying island nations in the Pacific and Indian Oceans.

Before I opened your thread, my first thought was glaciers! I'm glad you've seen some.

I agree with Venice due to the "sinking thing" or rising sea.

You're a lot younger than I am so my list is more....what to see before I "age out" of solo travel to Europe, lol!!

Coral and Manatees are both excellent...

Some Alaskan glaciers are growing, some receding, so I'd say see the lower 48 glaciers (almost gone) or the European glaciers (many in retreat) over the Alaskan ones.
At our age, most stuff will make it well past our expiration point, but your 25yr old may not have those same options, so you are smart to get those for her sooner rather than later.
But, I shortened my reply to a few:

• European glaciers and the few left in the Lower 48 of the US.
• The Amazon rainforest
• African safari
• Native American historic sites including Machu Picchu but countless others in the US, Mexico, and Central America as well
• US National Parks (they're staying, but are more crowded every year)
• Art masterpieces wherever they are - none are safe from bad luck or bad intentions (as the Notre Dame fire, the wars of the past century, or various art heists have shown us)
• Whales in the wild - some are coming back (yay) but some are still struggling (same with the manatees and reefs you mention)

I do hold out hope that all these and other things offered in other comments will last our lifetimes, there definitely is a "no time like the present" or an "if not now, when?" feeling for me these days.

Taiwan is high on my list. We'll see how that goes.

For really healthy, vibrant coral reefs, you'll have to make a significant investment in time/effort/money to reach them - and go much further than most people realize (I've been scuba diving for decades, and the decline in marine ecosystems anywhere nearby that gets more than a few tourists is impossible to not notice, it's quite dramatic). If you really want to see pristine, vibrant coral, I'd head to remote places in Indonesia - Raja Ampat is a good place to start (or other, even more inconveniently-located remote places). Alas, this will not be quick, easy or cheap (which is one reason they're still so vibrant: all the places that are quick/easy/cheap to get to suffer from being quick/easy/cheap to get to).

If you're serious about reefs, you and your 25-year-old will want to consider getting certified for scuba if you're not already (honestly, how long can you hold your breath? and even if you're a champion breath-holding freediver, the good stuff is too deep to see just snorkeling, and you're going to want to look at it for longer than you can hold your breath...so again, that's not quick/easy/cheap, an understatement...but you did ask...)

As a corollary to "see them while they still exist"  there's also "see them while they're not yet overwhelmed with crowds".  That one's pretty tough, too. 🙁

Manatees might be easier than you think. Check out Florida, I believe there are still some places where you can find them.

Whales in the wild? Good news, actually that's quite easy. You just need the right place/right time combination, and/or a little luck - with wildlife, luck always helps (how CLOSE you get to see those whales is another variable).

For very reliable and pretty-darn-close whales, try Baja. It's quite predictable (in multiple locations), not super far, doesn't require changing your life or making huge sacrifices in comfort/conveniences/financial ruin. It's completely seasonal, though, so you absolutely have to be there at the right time (this is good, that's what makes it so predictable). And it's even one of those rare, feel-good success stories, the rare case where things are actually and clearly getting better because of what people are doing (gotta savor those wins when we can).

This one is low-hanging fruit. PM if you want specifics on whales in Baja. 🐋 🐋 🐋

Polar bears (with a reputable guide company), sections of the PCT (becoming more and more difficult to hike the whole thing), African safaris (sustainable), temperate rain forest (Pacific Northwest), tropical rain forest (South, Central America or Africa), monarch butterflies, tigers, orangutans, the Arctic, the Antarctic.

Swimming with the whale sharks in Oslob Cebu was an amazing experience. Whale sharks are disappearing, and we need to do more to protect whale sharks before future generations lose the chance to marvel at them. 🙏