Killing baby Hitler is a straight-up dumb idea - I've talked about it before:
That's just the reality of the situation.
What you COULD do, however, is get Hitler into art school. Adolf Hitler was a moderately-talented artist with grand ambitions to attend art school - but his application was rejected twice, yadda yadda yadda, Europe was obliterated and tens of millions died in a variety of horrifying ways. So let's fix the unfortunate consequences of angry, nationalist leader Hitler by transforming him into gentle professional artist Hitler by just GETTING HIM ACCEPTED INTO ART SCHOOL. Whether that means bribing a school administrator or just swapping his application into the "accepted" pile with no one noticing, it doesn't matter - you get Hitler in, he doesn't become an embittered sadistic hyper-racist nutjob in charge of a nation.
Of course, this could lead to ANOTHER figure in German history rising through the ranks and leading a fascist revolution in his stead, but we'll just get that guy into dental school or something.
Listen, stopping 9/11 would be VERY complicated. We even made a video about it:
There's just A LOT happening in and around 9/11 for one random individual to stop it - so you'd be better off going a bit further back and taking out al-Qaeda's queen bee before the organization can even really get started: that's right, ruin the reputation of Osama bin Laden by drugging him and filming him having sex with a Jewish dude.
See, it's not enough to just KILL Osama bin Laden - if you do that, he basically becomes a martyr and his cause will live on. You will immortalize him and his ideals - but if you expose him as a hypocrite and a disgrace to everything his stands for, the entire cause will fall apart from the inside out. And the way to do that is to go back to Al-Thager Model School in 1975, lace his drink with ecstasy, and get him to have intercourse with a Jewish male (either yourself or someone you've hired for the task), filming the entire encounter.
Anyways - then you either wait around or time travel forward to the Iraqi invasion of Kuwait in 1990, where bin Laden and al-Qaeda began making a name for themselves, but before they'd done any real damage or started their enmity towards the United States and other western nations. And that's when you release the tape to every news organization and outlet you can find.
bin Laden will be shamed by his own people and his own organization - using drugs AND having homosexual intercourse with someone of the Jewish faith? Osama bin Laden will be strung up and killed or exiled by his own people, and al-Qaeda will implode from within, not only preventing 9/11, but multitudes of attacks that took place over decades.
Here's the truth about going back in time and seeing dinosaurs - dinosaurs likely sucked and looked like goofy birds covered in feathers, all of them would probably kill you at a moments notice so you'd die pretty quick, AND you're far enough back in time that your actions could wildly alter the future of the world and even threaten the existence of humanity itself. In short, DON'T DO IT, YA DUMMY.
What you SHOULD do, if you're sooooo into dinosaurs, is just go back to 1992 and become a PA on a little Spielberg film called Jurassic Park. Think about it - you get to see some badass dino animatronics, hang out on the set of one of the most ridiculously great movies of all-time, and see Jeff Goldblum shirtless at the time when he was at his most ripped.
Plus, you can warn Goldblum and Spielberg against doing Jurassic Park 2: The Lost World.
Saving Kennedy wouldn't be THAT hard - do something to delay him from arriving in Dallas that fateful day in November, or even find Lee Harvey Oswald and stop him from heading up to the book depository (assuming you believe the single-shooter theory, which you should, because c'mon dude). But Kennedy did more as a symbol than he ever really did as a president, and his death (and the resulting goodwill it created) helped push through the Civil Rights Act of 1964. Had he lived, much of what LBJ was able to accomplish in his years as president would have been impossible, and civil rights in the United States may have been set back years.
So what should you do? Well, maybe you remember a crazy ridiculous thing Donald Trump said during this election...okay I get it I need to be more specific: he literally implied that Ted Cruz's dad was involved in the assassination of John F. Kennedy.
Yes, this is a thing that happened - and then Donald Trump went on to win the Republican nomination AND receive Ted Cruz's endorsement, EVEN AFTER TRUMP SAID HIS DAD WAS INVOLVED IN THE ASSASSINATION OF JOHN F. KENNEDY.
Of course, this is patently ridiculous - the whole thing is based on a National Enquirer story claiming there's a photo of Rafael Cruz (Ted's dad) hanging out with Lee Harvey Oswald months prior to the assassination. But here's something fun you COULD do - get Rafael Cruz's autograph at the bottom of a sheet of paper, put the paper in a typewriter and write out a letter that says "DEAR LEE, THE BOOK DEPOSITORY WILL GIVE YOU THE IDEAL VANTAGE POINT. GO THERE, TAKE OUT KENNEDY, AND GODSPEED. YOUR FRIEND," and then it will look like Rafael Cruz signed the letter in his own handwriting. Open a safety deposit box in the name of Lee Harvey Oswald right before the assassination, and BOOM! Years later, when everyone's ripping on Trump for making up insane and ridiculous stories about his political opponents, you put in an anonymous tip of a safety deposit box, and wait for every news organization in America to be:
Completely baffled that Trump was right about his most insane claim to date
Blown away by a NEW WRINKLE in the oldest conspiracy theory-laden event in modern history
CONFUSED AS HELL
It would be very funny and weird and - hey! - it would help permanently sink the political career of Ted Cruz, which is definitely a plus.
Winning the lottery isn't all it's cracked up to be - most lotto winners end up broke and deeply in debt, it messes up your romantic life permanently, it screws up friendships, and you become a target for lawsuits. Basically, it draws WAY too much attention to you in a lot of bad ways, and you probably wouldn't know how to deal with that kind of money anyhow.
So what should you do instead? Win the lottery for ALL OF MANKIND by delivering the finished copies of A Feast For Crows and A Dance With Dragons (the 4th and 5th books in the book series, A Song of Ice and Fire) to George RR Martin in August 2000, right after the publishing of A Storm of Swords (the 3rd book in ASOIAF). Mr. Martin was on a roll with the first three books in the series - each releasing 2 years apart from one another. But the writing and publishing of the 4th and 5th books were legendarily difficult, originally being intended as a single book but getting so large and unwieldy that Martin was forced to delay publishing and eventually split the books in two. And even then, it took him 11 years to get through finishing up those two books (whereas the first three books combined only took 6 years).
Giving George RR Martin an 11 year headstart and allowing him to skip over the two most difficult books he was writing (particularly the notorious Meereenese Knot) would allow him to get right into the 6th and 7th books in the series, which even now in late 2016 are looking potentially questionable. And with Martin's age and the popularity of the show based on his works (Game of Thrones) causing further distractions (between awards shows, additional convention appearances, etc.), he may never finish the books.
But if he was able to start on the 6th book (to be titled The Winds of Winter) in late-2000, it's entirely likely that the series would be COMPLETE as of now, and Martin could go back to whatever other stuff he'd rather be doing, like blogging about the Jets or whatever.
Of course, like almost all time-travel stories, all of these supposedly "good" ideas would likely cause tons of unforeseen consequences and fallout and the world would be an infinitely worse place in ways no one could have predicted. So maybe the lesson is - don't use time travel, unless you're Marty McFly and want to invent rock 'n roll.