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.