

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