Trauma medicine and modern agriculture, the basics of modern scientific philosophy and dialectical materialism, I could probably draw a mostly-accurate map and chart a few of the notable dangerous currents, the dynamics of climate change/public health to get them away from fossil fuels.

I wonder if there's something that one could teach Homo erectus 3 million years ago that would permanently fuck up the course of history that led to the emergence of modern humans.

you are you ain’t making a steam engine the in bronze age

The romans built at least one.

Germ theory and using alcohol as a disinfectant. Even if you can't prove the science without microscopes or whatever, being able to make people not get infected wounds and die is both beneficial and doesn't require a huge baseline of technology.

Try and turn Napoleon into a Marxist

Going back to the early fifteenth century to tell indigenous people worldwide to use fire arrows against every European ship they see and to organize regular patrols to keep an eye on the ocean at all times. You can also inoculate people against smallpox just by sticking dried smallpox sores into their bloodstream. (The Chinese, Turks, and Africans already know about this at the time.) These two cool techniques could save tens of millions of lives and destroy the historical nightmare the world has been experiencing for the last five centuries before it even begins. Major issues are: organizing people and learning their languages.

I have spent several weeks at this point in my life fantasizing about what would happen if I were to introduce electric guitars and indie rock music to Weimar Germany.

Like, open up an actual underground club and start a band to play like, the Killers music or something in 1926 Cologne. I'm sure people would dig it, but how would it fuck with their lives and maybe history?

just binge-watch Cody's Lab first

I don't know how true this is but I think a lot of seafaring cultures didn't understand how you can sail into the wind (tacking). I mean you could probably go back in time thousands of years and show folks how to add a keel and how to point your sail correctly, no real "tech" needed.

hmm, i'd like to think that if you dropped me into islamic golden age, i could give them an insane boost in math and physics.

Inoculation is an easy one if I could get someone to believe it, as well as whatever basic medical knowledge I have on hand (cool it with the bloodletting maybe?). Might be able to make a simple battery if I thought hard enough.

Could do a crude musket if I could figure out gunpowder (can remember saltpeter and ash(?) off the top of my head). That opens up cannons too. Assuming I've traveled to before these were readily available, whoever I swear allegiance to could have access to briefly unmatched firepower until the technology was copied. Crossbow should be possible as well, and also doesn't need as valuable of resources

Outside of the 20th and 21st century, I think most things should be replicable pretty far back, like into the classical age if you're somewhere fairly "developed" like Rome. It's hard to underestimate the importance of simply knowing that something is possible because it was done before.

There's the time traveller cheatsheet, hopefully I'd remember it. https://i.imgur.com/dgJ7vHU.jpg