I’d print out all the correspondence between my ex-wife and I during our split up and divorce, then go back to 1998 to hand it to past self.
I’d go back t othe hospital in December of 1988 and carry my Dad back with me. He passed away after a triple bypass.
Lottery ticket. I don’t want to have to memorize a lot of new kings when I get back!
Yeah, same here. I was going to say that if I could do it repeatedly, I’d never again suffer from “staircase wit.”
Lottery, sure.
Or maybe something involving an about a 20 yr old Natalie Wood. 
Yes, but think how many kids might never have been born if WWII hadn’t uprooted half the population of the world and thrown countless people together who might otherwise never meet. You could be preventing your own conception by killing Hitler – and then who’s gonna go back and kill Hitler?
Same here. Altering history is too susceptible to unintended consequences (eg, what if Hitler’s successor actually succeeded) ?
I actually wouldn’t try to win the most recent massive MegaMillions lottery…I wouldn’t want to mess around too much with stuff that has “already happened.”
So I’d wait a week or two and see the winning lotto numbers for a smaller jackpot (I’m not greedy, a “few million” is more than enough for me), like a Powerball or something, and then go back a day and purchase the ticket.
Goodbye, student loan payments!
Count me as one who thought I could go back 15 minutes in time. In that case I would do nothing different. I have no regrets!
ETA: Since lunchtime, anyway.
I’d do the smaller (10 million or so) lottery thing.
Steal a few dinosaur eggs.
What’s all this talk about killing Hitler? You could go back and kill randomly! What are the cops gonna do? Hold you in jail for more than 15 minutes?
I’d either go back and pick the winning numbers for the recent Mega Millions lottery, or go back a few years, buy up a whole bunch of Apple stock, get the certificates, and put them in the safe deposit box I had back then (which I still have now). Go back to now, retrieve certificates, cash them in, profit.
Though that might be hard to do in 15 minutes, come to think of it. So I’ll go with the Lotto numbers.
I have no idea where I got the 15 minute time limit. It was just a thought that occurred to me this morning and I found it interesting. It does add that layer of urgency, though, to whatever it is you might want to do. Might make a good basis for a short story or something.
So, watta you doin’ with the other 13 minutes? 
I’d go back to summer 1984 and convince myself to ask that girl out. I came so close in real life, it was on the tip of my tongue, and maybe a note the day before would push me enough to do it. Lottery winnings are nice and all, but this is bigger.
Yup, lotto. I could probably make just as much difference with an assload of money, and still have some left over for assload of money things.
It’s worse than that. In a world where both Hitler and Stalin are dead in 1913, by 1945 all the winning lottery numbers are going to be completely different. Games of chance are especially vulnerable to the butterfly effect.
As for me, I’m not sure if I’d try to change anything - maybe just go back to observe, or bring back some valuable lost historical artifact. Hmm…
Presumably you can’t convince anyone that you’re from the future, except maybe for your (younger) self. And some crackpots over at the local science-fiction club.
I find it sad that we can’t come up with an objectively beneficial thing for all humanity.
Kill a key person of the Manhattan project, like Oppenheimer or Szilard? Sure, the Cold War would have been different, but not necessarily better. And the bombs would have been developed a few years later.
Warn the Titanic lookouts? A few thousand people would have been saved on that day, but the British ships would have continued to sail into iceberg fields with insufficient numbers of lifeboats, and eventually somebody else would have died.
Delay the launch of STS-51-L ? (Possible, I guess, with a bomb threat or something.) Then the shuttles would have continued to fly at a furious pace in all kinds of weather, and a similar accident would have happened a few years later.
Prevent the foundation of Islam? of Christianity? of Judaism? of Mormonism? Some are easier than others, but it’s not clear how things would have evolved.
Kill L. Ron Hubbard. That one’s pretty positive, with no downsides I can think of.
Tell my brother to go to the hospital before he dies of a pulmonary embolism.