what's a cool gift? (like an infinity mirror)

Why no one has mentioned a lava lamp?
A useless machine
I once had an hour glass that instead of sand it had some sort of slime, it was very cool to watch the bubbles slowly rise up the column.

Dichroic glass

Splits a beam of white light into different colours, and shines them out diffferent orthogonal faces of a cube.

Turn music down

If you're wanting something that makes for a neat, mesmerising visual effect, maybe a plasma ball?

https://en.wikipedia.org/wiki/Plasma_globe

It looks like they're available on Amazon for the mid-to-low end of that price range.

Little piece of bismuth was a good gift I gave, small desk ornament or such

This thread just walked me back into Sharper Image circa 2003

RGB LED strip kit. Can use as backlighting for a mirror or tv, and could later be used to make an infinity mirror.

https://a.aliexpress.com/_m0Lnqad

I'll list a few that I like but with no links because I don't want to use Amazon and most results that I find are limited to EU based shops anyway so not sure if it helps you. But I am sure you'll be able to look them up

Sand pendulum. Basically a pendulum that draws patterns on very fine sand. If you see them you can probably make one yourself that is even cooler

Double Pendulum, pretty cool to look at

Gyroscopes.

Tensegrity models. Those floating style structures that actually hang

Decorations with magnetic sand or fluid

Euler’s Disc, along with the most solid glass mirror you can find.

https://youtube.com/shorts/Q8M3Il0Zv68

The drinking bird

https://en.wikipedia.org/wiki/Drinking_bird

Those little parabola mirrors that make the toy seem to float above.

Newton's cradle

Good quality Rubik's cube

Prism color cube

A desktop stirling engine makes for a fun kinetic sculpture. They're powered by temperature differential such as by setting it over your morning coffee/tea or just the warmth of the sunlight hitting it.

Here's a fun video talking about stirling engines

Rock slings are incredibly fun, you can chuck rocks and balls so far people love to try it, especially kids

I've always thought Galileo Thermometers were pretty cool.

Arc electric lighters

Magnetic sand hour glass

Socks.

Awesome, and you can get them in different colors, like grey, or black, or blue, or beige. Not white though. White socks are only for playing sports.

If the recipient is technically inclined, then a kit for a mini tesla coil would be cool

Gonna suggest a kinetic sand table. The tabletop is glass and underneath there's a bunch of sand and a metal ball that rolls around drawing patterns. They're pretty neat.

I’ve always liked those magnetic levitating globes!

(You can also find magnetic levitating for a variety of tv shows and video games)