Go forward in time, build a giant freakin’ robot with a laser beam on its head, then skip back to 1938, just in time for Orson Welles to do his thing.
I’d leave an empty condom wrapper somewhere on Tranquility Base where Neil and Buzz couldn’t help but find it.
What’s so funny about Anne Boleyn in latex?
Actually, here’s a time travel prank anyone of you could pull now. It would help if you had like a lab coat, or maybe a jacket with a nehru collar, or some garment that was just a little off. And a pen-filled pocket protector.
Properly garbed, just run into a store or a bar and very excitedly ask the cashier “What’s today’s date?” After they tell you, “It’s September 12th.” you reply, “No, the year, dammit, the year! What year is it?” And when they tell you “It’s 2013.” you cry out, “IT WORKED!! IT WORKED!” and then go running joyously down the street exclaiming “IT WORKED!! IT WORKED!”. Until you round the corner. Then you can catch your breath, pull yourself together, and look for another store or bar. 
If you could add in something like “President Clooney will have to listen to me now!”…
Fill an easter basket and leave it just outside a certain tomb on the outskirts of Jerusalem around 33 AD.
Or a couple of days earlier: rubber nails. 
Let the Lascaux artist(s) see a T. Rex. Or a Teletubby.
Maybe sneak in to King Tut’s tomb a few days before it is discovered and put spring loaded fake snakes in the sarcophagus, or put a bunch of cigarette butts and candy wrappers at the top of Everest for Sir Edmund Hillary to find.
Travel back to my relatives lily-white southern baptist church and tell them that the end is near. Exactly one year way, in fact. It turns out that in order to get into heaven, you have to be able to A) dance really well and B) hold your liquor. The dancing has to be expresive. No square dancing, line dancing, foxtrots or waltzs. You gotta shake dat booty! Or burn for eternity. The booze one means that you have to be able to drink prodigious amounts of distilled spirits without passing out. Come back every month and watch the progress of the faithful.
I was thinking spell out “Kilroy was here” in Godzilla footprints for them to see.
One day after that, Gorilla Glue that stone in place.
Replace the stain on a certain blue dress with DNA from somebody else.
Remove Dead Sea Scrolls, replace with Book of Mormon.
Go back a few thousand years and place partial boat frame and the feces of a couple of hundred species of animal at the top of Mt. Rainier.
Go back to 150,000,000 BP. Capture plesiosaur.
Go forward to early 20th century Scotland (Loch Ness). Release plesiosaur.
And an outboard motor.
Go back to antient Egypt, Mesopotamia, China, and the Aztec kingdoms and carve images of a bowl of pasta with,meatball eyes in their most sacred temples.
Go to San Salvador/Watlings Island, approximately 1392. Gain trust and awe of local people with antibiotics, fireworks, digital watch, firearms, laser pointer. Then teach them Japanese (spoken and written), leave assorted early Muromachi period artifacts all over the place.
Cocaine mummies. Also, the Antikythera mechanism. :lol:
In some large cave I know from historical records is going to be discovered within the 20th century, I place the skeletons of a horse, a man in ancient armor, and of a Tyrannosaur with a lance through its ribs. All can be carbon dated as having died in (apparently) AD 300 or so.
I’d find the next shipwreck over and put an iPad in there.