How about a close-up of a character holding what appears to be a boulder over their head, then a zoomed-out image to show that they're holding an entire mountain? I've been toying with that for one of my characters for a while.

There was also an old comic where Superman showed off his strength by doing a headstand, but then you find out he was actually lifting the entire planet. But I'm not sure how you could do that in image form without a lot of text (in the comic they had to do a bunch of panels of scientists reading the richter scale/orbital readouts and what not)

Maybe breaking something in their hands. Like bending a giant metal beam or crushing some durable material.

A character could also use their other body parts to deform/break materials like metal or stone, like a female char crushing something between their boobs, or a character deforming a thick metal plate with their booty etc.

They could also do something like flick away some very large/powerful thing with one finger.

Besides lifting, that's some displays of strength that come to my mind.

As an aside, I'd definitely love to see you draw more impossible feats of strength, I really like that stuff~

Some of those ideas could work. =3
I have had a thought of a character needing to get on the other side of a HUGE steel door, and they just flick it open.

But yeah I can see some of these ideas working out. =3

guess it kinda depends on what you have in mind, mean know you mentioned something like an aircraft carrier being out, but what about denser things than what would be considered traditional? would end up depending on the context and the likes, could be something as simple as anchors for something normal, or going to extremes and using things like balls of dark matter, n whatever else the main can think up :p

Obvious suggestion:
https://www.youtube.com/watch?v=5lxNNlQYttU

Maybe someone crushing an entire planet between their thighs?

I'm not into the destruction aspect like that, and not into macro either. =P

Normal looking dumbbells with implied ultra dense metal and labeled in the tons range.

That is indeed a possibility. I guess I just wanna avoid the same thing I did before.

coming out of a jail cell by bending the metal bars

the great escape from horny jail XD

Strolling down a railroad track with a rope over her shoulder, hauling a train behind her? (long string of boxcars shouldn't be too hard.

Similar to Mr. Incredible in The Incredibles, that could work. =3

Just a weight

How about anchors at a beach? or a trophy fish ?(really big sword fish), or pulling a ridiculously big chain?

Condense coal into diamonds with their hands, tits or whatever.
Tripping into an elbow drop that cracks a planet.
An homage to the time Superman carried a bunch of planets on a chain
Cranking gravity in a room to extreme amounts that her weights are pancaked comedically flat.

Pushing something incredibly big and heavy. You could even go silly with it, like them pushing a mountain elsewhere, or widening a small space between two buildings by just pushing the buildings further apart. Looney Toons would be a good source to find silly feats of strength in.
Or here's a fun idea. The strong character is doing push-ups, but they have another character with massive assets on their back. You could just play around with the idea of lifting hyper-sized characters in general.

She lifting another hyper character xP amd maybe that character being some overweight one

2 Marshmallows on a stick

I don't know if this counts, but maybe you should try drawing someone flexing one bicep to the extent that it swells to astronomical proportions. You know, kinda like in this picture, but with a lot more magnitude: https://www.furaffinity.net/view/25085715/

I realize I'm a bit late to the party, but I love impossible feats of strength so much that I couldn't stop myself from wanting to contribute!

I personally like the thought of Ophira walking through some wall with her ultra-narrow waist, which turns out to be so narrow that she splits an atom, setting off a nuclear bomb but is ultimately unharmed.

Alternatively, doing some feat of strength in her sleep or multitasking in general is a somewhat easy way to make a feat more impressive.

Not sure how easy that first idea would be to draw, but I can see drawing her with an even thinner waist. =P

Winning a tug of war with a supermassive blackhole