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.