One low-hanging fruit would be sterilization and hand-washing for medical operations, at least.
Significant steam power (so not that ancient Greek toy) might prove too difficult for the engineering of the era, but a pressure cooker might be possible as it’s allowed to leak.
Screw cutting lathes, and in particular the guided toolpaths to make the output reliable and consistent, would be a big deal.
Might be able to bootstrap enough magnets and wires for basic electricity, at which point you can make much better compasses — the Chinese were the first to go beyond lodestones and that was about 1000 years ago — and electricity makes electroplating possible and some acids (e.g. hydrochloric) and alkalis (e.g. sodium hydroxide) basically trivial.
Float glass would radically increase the size and quality of individual windows panes. Knowing that lead oxide reduces the melting point would make manufacture much easier.
Wikipedia’s list of medieval technology has some interesting surprises: apparently wheelbarrows are only about 850 years old, hourglasses and segmented arch bridges only about 680 years old.