What normal household item becomes a mythical magic item when given to someone in a fantasy novel?

AHEM. *Cough-cough THE RAPID-FIRE WAND.

Lots of small tech, but if we want it to stay useful to them longer than one battery charge, I'd say a crank-powered flashlight or lantern.

I suppose anyone doing a lot of math, like a moneychanger, would also lose their shit over a solar powered calculator.

A solar powered calculator, that one I like.
That is simple enough that it could actually keep working through the isekai teleport, and it also probably would comport with the arithmetic knowledge of the smartest person in the village, and might not get you immediately killed as a witch or warlock.
8008135
- 04 55378008

I'm not sure exactly what you mean.

If you're asking what household item doesn't actually change, but would be considered extraordinary by someone in a medieval setting, and function and be useful in that environment, I'd say matches.

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

It sounds like China had a primitive chemical match well before Europe, but that it wasn't until the early 1800s that Europe had the match in common production, so I'd guess that a European fantasy setting likely wouldn't have matches.

Oh, another one. A stainless steel knife. Stainless steel apparently didn't exist until the early 1800s.
https://en.wikipedia.org/wiki/Stainless_steel
"The knife that does not rust."
It sounds like we didn't have aluminum until the early 1800s, either (and it was very expensive for a while, until we got processing with electricity), so very lightweight metal objects would be pretty remarkable.
https://en.wikipedia.org/wiki/Aluminium
The wandering inn has a couple chapters (and even starts a gang war) over matches.

The Plants that Would Not Die

Light at the flick of a switch.

It's was a glacially cold moonlit night. Our hero turned on the LED ceiling lights.

Which drew their power from...?
- Divine intervention

Polaroid, lighter, vape, flashlight, fleshlight

🎶 ...we didn't start the fire! 🎶

gun

I'm just imagining you showing someone in a fantasy realm a modern Glock and they go, "Ah that's like our matchlock pistols!" And they explain the dwarves and their technical capabilities.
- I forgot about those dang crafty dwarves! If Mr. Gun (the glock) fails to impress, it's time to whip out his papa; Dr. Gun (something big and fully automatic idk gun stuff lol)

Slinky, vibrator, laser pointer

I want a Isekai where the person has a laser pointer.
The nearest thing is Dr. Stone and the Mentalist.

The standard faire would be a mirror or the ladder to the attic or a broom.

If you’re looking for the path less traveled, a microwave magnet is a fantastic way to curse your nemesis’s compass.

A microwave without an accompanying, compatible power source would just be a weird box.
EDIT: Oh, you said the magnet from one, I can't read lol.
Well, uh microwaves have electromagnets.
They don't produce much of a magnetic field without a power source.
So... same problem, actually.
- I cast one of my donut shaped microwave magnets in your direction. Qty: 30
The magical item that confuse the monster's sense of direction.
And also can instant kill them.

Brooms

Stick of deoderant.

People tend to very much not consider how fucking awful everyone smells when almost everyone is a manual laborer, washing machines don't exist, indoor plumbing largely does not exist, people tend to have a wardrobe that can fit in large satchel, not an entire walk in closet.

This is part of why people really really viewed annointing oils and incense as luxury items.

Just mask the smell lol.

EDIT:

Any kind of memory foam anything, pillows, mattress.

Totally impossible to produce without modern industrial society, heavily reliant on modern chemistry, oil processing, etc.

The level of comfort would likely be seen as literally supernatural.

Also:

Zippers.

Yep, you heard me right, zippers.

Sure, you could have a finely skilled artisan who crafts mail or scaled armor.

... Zippers?

Real ones, made of metal?

Yeah, the not shitty ones are made of metal alloys that roughly medieval people could not create, not to mention they are made of very small pieces that must be pretty precisely shaped, and then also all finely and sturdily woven together.

If a handheld clock is basically magic in your scenario, a zipper is almost as hard to craft, but has a much, much broader range of practical applications.

And if its made of plastic?

Again, plastic requires oil refinery. Plastic = Magic.

Yeah, theres a reason why most pre industrial fashions use folds and drawstrings and buttons... and not zippers.

EDIT 2:

Maybe not common household items unless you are a bit of a prepper, but uh:

Mylar (space) blankets / ponchos.

Emergency water filtration 'straws'.

Sanitary wipes / individually packed steri-wipes with alcohol for cleaning wounds.

- Fine and good points!
  Yes, we do tend to have much shitter quality fabrics these days, at least for the masses... as you say, medieval era common clothing did not need to washed as often, tended to generally be more rugged, and also were more easily capable of being repaired.
  Myself, I hope that at some point we go back to hemp based fabrics, or at least include hemp in the blend.
People tend to very much not consider how fucking awful everyone smells when almost everyone is a manual laborer, washing machines don't exist, indoor plumbing largely does not exist...
This is a common myth but people absolutely did wash and do their best to stay clean. Being smelly was just as undesirable as it is now, if not more so because it was commonly associated with disease. People would have a ewer (pitcher) of water and a basin in their bedrooms and wash themselves every morning and evening with a cloth. Of course scented oils and perfumes used to be a huge business, but even people who were extremely poor could make or buy "posies" or sachets of sweet smelling herbs, grasses and flowers and carry them with them. As another poster said, they would also wear natural fibers like linen, hemp, cotton or wool that don't hold on to scents as much as polyester or rayon do. Even if they had one set of clothes, they would have at least two under clothes like shifts or shirts or tunics and wash them regularly in ammonia and hot water.
Deoderant would probably be a neat trick and on par with an expensive perfume but I doubt it would be on the level of magic.

a stove, or perhaps matches. they'd allow you to use fire magic without needing to know fire magic.

A Cup, 3 pieces of jewelry, a diary, a pet and for some reason an opponent's child.

Hair dryer

a labubu~

or your superbly crafted resin figurine

for household items, i think the ceramic knife hits the spot.