Introduce Shakespeare to D&D, and encourage him to popularize it.

Not only would the campaigns he ran be amazing, but goooodlord imagine the subversive effect it would have socially. Unpinning good/evil from lawful/chaotic in the public perception that early on would be a Big Deal; bringing the idea of consumer-generated content would shift attitudes to art and literature away from a purely top-down concept, and the resulting rise of Victorian fan-fiction would be so amazingly terrible it would rip its own hole in the spacetime continuum.

Id show up at the time travelers convention.

give my bro harambe a bullet proof vest

The first time Heinrich Kramer tries to show someone the Malleus Maleficarum, I appear directly in front of him and set the book on fire. Not only is the book destroyed, but a clearly supernatural event took place to put the fear of god into him. Bam. No witch trials.

I'd prevent the Challenger launch. Manned spaceflight doesn't get shelved for an entire generation, and a young me doesn't lose hope for the future at such an early age.

Through a bizarre series of butterfly effects, the successful launch and its international attention gives bureaucrats in Pripyat an extra nudge to encourage cooperation amongst their engineers and nuclear scientists, and a critical flaw in the operation of the plant at Chernobyl is caught before it causes a catastrophic meltdown.

The cumulative effect is a continued culture of progressive technological expansion into the 90s, and the fading of the anti-intellectualism that threatened to overtake the world during the Reagan and Thatcher administrations. Hand in hand with this is a decreased militarism, as technology is increasingly seen as a tool for the betterment of humanity, and less as a means of building better weapons.

One other immediate result is in the US presidential election of 1988. A lack of meaningful engagement with the public (no "skipped the surly bonds of earth" speech) led to increasing apathy toward the outgoing Reagan administration, giving G.H.W. Bush a tougher hill to climb, and less solid footing on the issue of defense. Dukakis doesn't feel the need to do a silly photo op in a tank, but instead campaigns partly on an expansion of the space program and educational outreach programs similar to the one that brought in Christa McAuliffe.

Neoconservatism and neoliberalism wither together on the vine. Permanent human presence in space continues uninterrupted for the next two decades, with a base on the moon by the end of the century and a manned mission to Mars planned for a decade after that.

No Bushes, no rise of Al-Qaeda in 1988, no Gulf War, no Rush Limbaugh, no Clinton's, and no 9/11.

I'd save RFK and give him a full two-term presidency. Just because I've always wondered how much a difference it would have made in the course of American history. It definitely seems like things took a severe turn for the worse in the late 60s and the American political system has never recovered.

I personally believe there were conspiracies to assassinate both him and JFK because they were not susceptible to being controlled by their donors or political mentors, as is the case with the vast majority of politicians. They were rogue elements with a strong potential to disrupt the status quo (i.e. gravy train) for the rest of the oligarchy, so they got taken out.

I'd go back and prevent the 11th Sept attacks.

The world would be a different place because so much happened as a knock on of that but at the same time it's hard to imagine what the world would be like. Probably very similar but also different in substantial ways.

Like obvious things like no war in Iraq or Afghanistan (at least not those wars), and less obvious things like how the attacks have reshaped liberal democracies like the US and Europe (for the worse imo), and how they empowered right wing politics in many countries (also bad imo).

A highschool physics book translated in ancient Greek/linear B to mass copy and distribute to everyone. Maybe it'll give the advantage to stop the Bronze Age Collapse.

Introduce radio to the Romans. They had the metallurgy to create coils. Even a simple Morse code system would easily keep their empire going. Probably end up like that Star Trek TOS where Centurions are carrying sub-machine guns, though. If want to read what a great SF writer did with this (guy from 1938 ends up in 535AD), read "Lest Darkness Fall"

Something that would do that neoliberism in the 80's with Reagan and Thatcher would not become the dominating political and economic theory it has been since that time.

Idk sneeze on a dinosaur or something.

If you're fine with utterly erasing pretty much every human being conceived after that point in favor of new human beings (or other creatures entirely) may as well go big or go home and start here.

I often wonder how people would react if you showed up to a concert hall in, say, classical music era Europe or something and performed modern music. Assuming you could kit it to provide infrastructure for whatever your performance required, and the acoustics of the venue were idealized.

Would attendees hate it? Would the unfamiliar musical styles be repulsive to them? Would the sounds and textures of modern instrumentation like electric guitar and synthesizer upset or even frighten them? Or would they find something to appreciate about it? Would the music be copied and spread, becoming a time worn classic folk tune in an alternate future? Or would it be rebuked and suppressed, condemned for all time as evil influence? Which genres would have the best acceptance chances in which cultures, and which eras?

In my mind in particular, I think about this with the niche realm of video game soundtracks. If not just the music played as-is through some playback device (which would probably be rather boring, but who knows, maybe the novelty of recorded music alone would be fascinating enough) then perhaps arranged for live performance, like the orchestral performance of Undertale, or the Sinnohvation big band album. Or, of course, if the soundtrack was itself a recorded live performance, just perform it. These collections of compositions often outline rich adventures, communicated by a wide range of musical styles. I wonder if they are strong enough to stand alone, and if audiences would respond to them without the context that they were written to accompany.

Failing live performance (which would be trickier than one would think--to sound good, live music has to be written with its venue in mind, and I'd assume most modern music would sound like garbage when performed in victorian era concert halls or ancient ampitheaters), I'd also consider putting them to vinyl LPs and dumping them in old record shops in any era that had phonograph or turntable technology and see if they get discovered.

Why not just send back the video games themselves? I dunno. I guess I'm less interested in wowing them with futuristic technology and more interested in how they'd react to something they already have (music), but in a strange, new context.

Start flamewars on robotic astroturf accounts about how dumb Donald Trump is until Instagram starts and people try to prove he's not an idiot, but in protesting they protest too much and nobody believes them by 2016.

So, I need a robot chatbot algorithm cookbook for the naughties and beyond.

Allegedly, when William the Conqueror first landed in England he tripped in the sand.

I'd've left a land mine where he was going to fall, turning the future king of England into a fine mist and a scattering of viscera.

Probably Sound of Thunder meself out of existence, but it'd be worth for the immediate chaos it would cause.

Id go back and push a kid into the gorilla enclosure at Cincinnati zoo, just for a laugh.

You wanna fuck up the timeline? Somehow mess something up so plastic was never, and never will be invented. That would change sooooo many things.

I'd swap some of the first clay documents around until I ended up with a timeline where we live modern life with a gift economy rather than a money economy. We'd all have a lot more options to pay off our debt rather than the streamlined ridgid money system.

I would bring a sandwich for Gavrilo Princip.

Take the current highest-yield nuclear bomb and destroy England right before the begin of their collonial era.

Generally speaking, I believe removing a global superpower just before they do their world-changing thing is probably going to have the biggest effect on the timeline.

Here's a darkly hilarious one, because you could do it by accident: give smallpox to the native Americans a few hundred years early. Right around 1000 AD, show up to shake hands and teach metallurgy or whatever, maybe planning to jump-start their resistance to colonization... and their resistance to slaughterhouse-borne diseases, the hard way.

This would of course completely fuck up their population numbers, much the same as would happen in Europe in the 1300s. But by the time Columbus showed up to be the absolute worst person who could possibly discover a new continent, they'd be largely recovered, and they'd get to trade whole new strains with the seasick lemon-sucking weirdos who kept asking where the spices were. The returning ships would offer tomatoes and potatoes and another Black Death. Hopefully preventing Malthus from being such an influential bastard, and causing the first engineered famine in Ireland, whose population did not recover from the potato famine until this century.

New England colonies would presumably still take hold, but wouldn't steamroll all the way to the west coast. Hopefully they'd be limited and northern enough that slavery is less prevalent, less absolute, and - ironically - still a matter of trade. Because what ended the triangle trade to America was the treatment of African captives as livestock to be bred. The politics of this alternate timeline would be hilariously complex compared to now, and probably result in more and stranger wars than we can imagine, but there would be so many averted times where atrocities happened, effectively unopposed.

Grays. Sports. Almanac.

Killing Bill Gates. It would be a weird timeline, maybe Microsoft would still exist, but in a different shape.

Phonographic recording would be incredible to introduce very early on. Basically as soon as the pottery wheel, you could have recorded and played back audio. They'd have to do some fractions-and-guesswork materials science to get anything properly reusable, let alone the ability to press new copies of a recording. But it would get done. Every ruler would want their voice carried to all corners of the territory. Musicians would be known to people who'd never see them in-person. We'd have so much more evidence of how languages were spoken.

But specifically - in the spirit of the question - I'd do this by taking back a crank-operated player and a whole stack of bagpipe albums. Just completely fuck up what ancient peoples think music is supposed to be. Accordions would also work, but I'm not sure I could construct one for demonstration purposes. I could build a hurdy-gurdy, at best. And nobody has a shelf full of vinyl for insufferable droning hurdy-gurdy music.

I would go back in time and arrange Genghis Khan to have a better father.