Talk Leonardo into painting big furry black eyebrows on the Mona Lisa. Because I’d be the only one who knew, and that would be hilarious.
Show up the exact moment Jesus (if there was such a person) died with an ambulance and a defibrulator. Or show up two-and-a-half days later and replace his body (when/if he gets up) with a RealDoll ™. No I’m not linking to that.
Scatter RealDolls (with the special mermaid modification (I’d have to special-order that one I think)) throughout the oceans for early sailors to find. This would also have interesting effects on various branches of science other than reproductive.
Show Eve the proto-human how to breakdance.
Take a big book of sports statistics back to the pyramid-building… uh… Incans? Mayans? Forget which. The guys who had that basketball-type game contest thing, you know the one I mean. Convince them to name their emperors after famous modern basketball players, and inscribe them in stone. Frequently. Watch as, year after year, new famous players’ names just happen to match these thousand-year-old engravings.
Set up some speakers behind “The Burning Bush” and wait for God to leave, then tell Moses what he’s REALLY supposed to be doing with his life.
Convince Buddah to eat right, and exercise.
Teach the native American Indians perfect modern english. Maybe with ebonics.
In fact, the modernization of these poor people (including immunization and refrigeration technology) has long been a fantasy of mine. I enjoy imagining the looks on the faces of the first white men as they confront technology well beyond their ability to comprehend, and actually having to make a FAIR offer for Manhattan (among other things).
Fill several of the crates of tea with Alka-seltzer at the Boston Tea Party.
Set up a Starbucks at the Great Salt Lake, opening date timed to coincide with John Smith’s arrival. Include alcoholic beverages.
Get Edison drunk, then instead of singing “mary had a little lamb” onto the first sound-recording device ever, just make farty noises with our armpits. Ensure that this artifact is preserved and makes its way into the Smithsonian.
Convince the architect of the Sphinx to add big bushy eyebrows, nerd glasses, and a mustache.
Give my 10-year-old self a fully loaded modern tank. And the keys.
Give the Venus de Milo a very faint, nearly indistinct, third nipple, in such a way that it becomes more distinct with time.
Convince Adolph Hitler that nobody’s gonna ‘get’ the swastika as a symbol, and that a much more effective icon would be the yellow smiley face: 
Transport breeding colonies of dodo birds to various suitable locations throughout the earth, ensuring they never become extinct. I have no idea why, it just seems like it would do SOMETHING to the future.
Teach the cavemen how to paint by showing them Bob Ross videos, projected once an evening onto a nearby sheer cliff face from a hidden location, over the course of a couple generations.
Introduce Gutenberg (the printer) to the wonderfully profitable business opportunity known as “pornography”.
Introduce Steve Gutenberg (the actor) to the wonderfully profitable business opportunity known as “pornography”.
Convince Jim Davis that the ideal cartoon trio is Cat, Dog, and Llama. Be disappointed by the fact that Garfield STILL isn’t funny.
Convince Jimi Hendrix to get braces. With the big headset. Just before Woodstock.
At the battle of Little Big Horn, right before the native Indians attack, lead a 50-piece highschool marching band across the battlefield. Wear running shoes and a bullet-proof vest. And an arrow-proof suit.
Get one of those pickuptrucks owned by the railroad companies, which have the attachment that matches the railroad’s tracks and allows the truck to drive on the tracks. Wait about 10 miles away from the point where “East Met West” on the day the golden spike was driven in, then drive up and ask which way to Texas.
Sneak in just before the Russians launched “Sputnik” and include an AM radio transmitter and an endless loop recording of Howard Stern on a common, but previously unused, frequency. Alternately, some nice Pink Floyd. Better yet, ALL Pink Floyd.
Purchase numerous helium-filled remote control toy blimps and make a list of reported UFO sightings. Step back in time, stopping at each one as you go back further and further, and ensure that SOMETHING is in fact seen and recovered at each and every reported sighting. The trick here is to make sure that each blimp is found BEFORE the previous one, so that today it would be easy to discredit the sighting… yesterday just as easy, a year ago just as easy. But ten years ago it’s a little bit weirder. Twenty years ago it’s downright odd. Fifty years ago, it’s barely recognizable as earth technology. Two thousand years ago, it IS an alien spaceship. Make sure to return to the present after each delivery, and collect the documentation. Then, when you’ve finally done every single one, come back to the present and open a museum.
…must… stop… too many… ideas!
