Me, I send a nice healthy stack of Downfall Youtube parodies to Hitler c. 1942, and then read about his death from a heart attack on that date in the history books. 
[Youtube took down the original Robot Chicken clip of the same name, or I would have linked to it]
Oh good lord, the title…
Well, I’m a hetero female, but if I were a time-travelling dick, I think Bridget Bardot’s shower would be a great place to start!
Show a bunch of people that I can accurately predict the lottery and then refuse to give them winning numbers. Or make them debase themselves competitively only to give them numbers that win 10-15 dollars.
You’re a Dick With a Time Machine-what do you do?
Ooh, ooh…grassy knoll. Definitely would revisit that area LOTS. Would get lots of gigs if I could solve that one!
I would arrange to be the one in charge of paying for the Alaska purchase. Then I would show up at the Russian governor’s house and dump a truckload of pennies on the floor, tell him “Here’s 7.2 million dollars! Now count them.”
Or go back with an iPhone and, the day before the announcement of the first iPod, hold a news conference in Silicon Valley showing this little device I just built in my garage.
How big a dick are we talking about?
The type that would tuck a few bibles into the carcasses of a few dinosaurs just to see the look on peoples faces where they’re uncovered a few million years later?
Or the type who might bring back a t-rex on set it loose in a kindergarten?
That gives me another idea. Every week during NFL season, send a letter to every major newspaper saying “Here are the final scores of this week’s games.”
I’d throw Leonardo DaVinci out of an airplane with a parachute strapped to his back. Let’s see him concentrate on the Mona Lisa now.
I’d go back and hide your TV remote…and car keys!
I’d go back to the late Cretaceous and step on a butterfly.
what, you want that upon return we’d be in the middle of the Cheney/Rumsfeld administration… and George W would be our leading linguist? Bravo.
I’d release a bunch of rabbits in the precambrian.
See I was thinking of visiting Hitler too, except I wouldn’t try to kill him or stop him or anything like that. Instead, I’d try to convince him to grow a chin strap beard instead of a short moustache (I’m not sure how I’d manage it but I’m sure I could leverage my advanced technology and knowledge of the future somehow). Then I’d go home and never have to see some fucking douchebag walking around with a chin strap ever again.
Leave Susan B. Anthony coins throughout all of time and space.
Oh, I can think of all kinds of stuff.
Plant a bunch of detailed, breathtakingly accurate, predictions in Nostradamus’ ramblings. Things that wouldn’t be changed by foreknowledge. Things like the exact time, date, size, and location of major earthquakes, hurricanes, volcanic eruptions, etc. Then, after a long string of these accurate predictions that should convince anybody of his abilities, insert a false prediction of the end of the world.
Plant a bunch of busted up 21st century tech at the site of the 1947 weather balloon crash at Roswell, New Mexico.
Screw around with early Christianity so that Santa Claus (toys, red suit, “ho ho ho”, belly, pipe, chimneys, elves, North Pole,reindeer , Rudolph’s red nose, etc.) and the Easter Bunny (baskets of candy, colored eggs, etc.) are actual Biblical canon. Maybe I’d also add a bit about someone’s grandma getting run over by a reindeer.
Go to 1621 Plymouth and change the menu so that tofurkey becomes the centerpiece of a traditional Thanksgiving dinner. (I guess I’d have to involve the Japanese somehow.)
Call myself “Bozo-Land Vespucci” and take the place of Columbus’ navigator.
I’d go back to 1983, join the first lineups for the new movie [del]Revenge[/del] Return of the Jedi, and casually say, “Of course you know, Vader is Luke’s father…”
Repeat for as many other movies, albums, etc, as I can manage.
Publish all of Steven Kings books a year ahead of him.
Wouldn’t they all have known that for three years already?
Of course they know it. They heard Vader say it in the previous movie, The Empire Strikes Back.
Nah, I’d take a 52" plasma screen and a copy of Episode III and show it to extreme fanboys a few days before ESB premiers. It would be the most awesome movie they’d ever seen but it would totally spoil EVERYTHING.
I’d sell use of my time travel machine to the highest bidder, once a week.