If I’m going back more than about 250 years, I bet I could figure out how to do the cowpox/smallpox vaccination. (And I’d be dressed as a guy anyway, because no way am I wandering around eighteenth-century wherever as a lone woman, so that should get me taken slightly more seriously.)
Anything mechanical, I’m bollixed.
Far enough back you can have an effect dead easy. There’s about 5,000 years between the domestication of the horse and the invention of the stirrup. Once my conquering hordes can stand in the saddle and strike down those who oppose them all who refuse to bend to my will are doomed. While we’re discussing horses, it took Joe Glidden until 1873 to invent barbed wire. Hell, a child could do it once you give them the idea. I’ll bet there are hundreds of ideas like that. Pencil erasers, Zippo lighters, felt tip markers. Nothing high tech, you could build it at home.
Nope! Shakespeare got to be Shakespeare because he spent twenty-five years as actor, shareholder, and house playwright for his theater company – a situation that gave him an unusual amount of creative control over his playscripts. He’s also SHAKESPEARE, as we know him, because his near-complete works were published in the First Folio seven years after his death, having been shepherded into print by his friends and fellow-actors.
If I memorized Shakespeare’s plays and went back in time, I couldn’t have the same type of career or the same stable relationships with theatrical colleagues that he did, since I’m female and can’t act anyway. What I could do is sell a playscript to someone like Philip Henslowe, if he were willing to buy it, but I’d be paid a pittance, and I wouldn’t have any control over what happened to the play after that. It might never be performed at all, or it might be performed after significant rewriting, and even if it did make it onto the stage, the odds are very good that it would never make it into print. Or, even if it were printed, there’s no guarantee that copies would survive into modern times. There are more lost plays from that era than surviving ones. (Shakespeare’s contemporary, Thomas Heywood, claimed to have written or collaborated on two hundred plays – of which twenty-some-odd survive.)
So I’d much rather let Shakespeare be Shakespeare. Less risk of accidentally losing his entire works 
Yeah, see - this is where everything founders ;). I hate to fight the hypothetical, but 99.999% of the time any hypothetical single time-traveler is not going to accomplish squat. Eric Flint sent an entire reasonably well-armed town back - that might work. But one person? It might be that rare youngish, athletic, renaissance man with great linguistic skills and robust health that makes it( and as per WhyNot, in most cases you would have to be male ).
If I accept the hypothetical, I’m not going to accomplish much or likely anything IMHO. If I realistically fight the hypothetical as above, the outcome almost certainly won’t be pretty. If as I am tapping on this keyboard right now I am somehow suddenly hurled back unprepared to Domesday England in 1086, I expect I will die a rather painful death in short order.
ETA: So make it hard - you know none of the languages and will have to find and build your own human support structure. You also drop in a lightly inhabited area bare-ass naked, with nothing but your mind. Now can you accomplish anything?
The cholera in in the water! The cholera is in the water!
burns at the nearest stake
If you mean using my mad techno-science skillz, nuh-uh. A lot of book knowledge, near-zero mechanical aptitude.
Hadn’t thought about that story in 40 years or so.
I don’t know how to do any technology
if I time traveled back a short time, like 10 years, I would be an expert on future internet memes
Sorry Nemo, forgot about your admonition in the OP.
Yeah, probably the best I could do is teach math. Or, if it’s pre-1500, maybe draw maps and beat Columbus.
There was a series of books about a time traveler named Conrad Stargard, who DID seem to know everything about everything, and started a bunch of factories in 13th century Poland. IIRC he was able to pretty quickly build airplanes and machine guns in time to stop the Mongol invasion. It’s been several years since I’ve read them, but I vaguely recall his protean knowledge as seeming harder to swallow than the time travel.
That’s the Leo Frankowski series I referred to earlier in the thread.
I think it would depend upon to when in time and where I travelled. Assuming I arrived safely somewhere in the Roman Empire (because I can just about remember my Latin) I could do things like promulgate decimal arithmetic, algebra, the scientific principle, heliocentricity, Newtonian mechanics, and basic hygiene. With the ear of the monarch, I could do things like promulgate roads to increase trade, sewers to keep cities healthy.
Wasn’t he an engineer?
Physicians and alchemists hate local time traveler for discovering this one weird trick…
I know how to fly a plane and something about aerodynamics. I think I could probably help out the Wright Brothers and other early aviators. My C programming skills are not going to be very useful though.
If you sent me back more than just a few decades, I’d probably be arrested and executed. Considering the plight of women and children in past ages, I’d probably feel compelled to crusade for better treatment and conditions. The local authorities would NOT approve.
If I could get the ear of a semi-powerful patron, I’d probably do pretty well. The trick would be getting the patron.
sadly, I’d be limited to the fashion industry, predicting clothing trends.
I’m going to bend the OP rules a little and time travel back to old Roman times, grab Jesus Christ and bring him back to the present. I’ll take him to a Catholic Mass and tell him he’s always going to play second fiddle to his mom postmortem, unless he pulls off something really spectacular. Then I’ll send him back to his own time after teaching him how to escape from crucifixion nail impalement, in a billowing puff of smoke, Houdini-style. That’ll go down more satisfying in history than that lame resurrection trick he pulled off.
You could invent a universal Turing machine.