Vaccination for everyone! Aqueduct (AND toilets) for everyone!
Make good publicity for Mr. Volta's new chemical battery, and convince everyone of how ugly the world is when tainted by coal smoke. This has a dual purpose: ease the way for early development of electric cars, thus fighting global warming, and delay Western meddling in the Middle East for oil extraction purposes, which contributed largely to the mess the region is now.
Find Mr. Heinrich Marx at his law practice in Trier and quietly castrate him.
Popularize DIY production of blue cheese and thus increase the chances that someone playing with Penicillium fungi will get creative.
Recruit would-be Temperance Leagues and redirect their strength to strangle the tobacco industry in its crib.
Edited to add: only massive distribution of aqueducts and toilets would be obvious to a true native of 1800.
Batteries still mean that you need electricity and that means burning coal.
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