“Mein Furher, Ich moven towardsen die Orc und attacken eet mit meine sword!”
“Holden zie on, vehr ist das gottdamn to-hit table! Sheissenbooken!”
Blam!
I’m so sorry, but I will never tire of that gag. 
325 AD Council of Nicea-
Canon of the New Testament- no problem (tho I’d recommend a secondary canon, consisting of the Didache & the Apostolic Fathers)
The Trinity, JC as fully God & fully man, the Creed- no problem
Enforcement of Sunday & the Easter dating & Church festivals & open-diet and forbidding Sabbath, Passover, Hebrew Festivals & Kosher dier- NOW HOLD ON THERE!
And also, Constantine, show me ONE VERSE in which JC or Paul condones State enforcement of C’tian Faith.
I would go back to September 11th and at 8 a.m. call in a bomb threat for the World Trade Towers, telling authorities to evacuate them immediately, the call DC and give the same threat to the Pentagon and Capital buildings. Or go back to around 3 a.m. September 11th and discharge a car bomb between the two towers and one in DC. Either way, fewer people would be killed.
I’d be tempted to do the lottery thing, as was mentioned above. Either that, or get some powerful weapons, and go to a period where there wasn’t gun powder. I could win some impressive battles, and become a ruler. Then I’d get a large harem and live a life of excess. Language barriers and such would be dealt with during that two year preparation period.
Another selfish one would be warn Lennon on the morning he was shot. Something impressive enough so that he would listen and get his killer off the streets.
As for something that would benefit mankind, I would have to go with the Library at Alexandria. I like that idea. Perhaps spend my life doing what I can to preserve valuble manuscripts for the future.
I would also like to go back to July 1999 and shoot JFK Jr. in the leg before he gets on his plane.
Of all the people you could save, why him? I’m not asking that derisively.
Definitely
Africa, around 1 Million BCE, in order to hunt down and kill every living specimen of Homo Habilis or Erectus or whatever Hominid was around at the time.
Hunting rifle & lots of ammo, night-vision goggles, pack mule, survival/hunting gear, poisons and plague germs.
Get myself immunized to the plagues I take back, learn everything currently theorized about the habits and locations of said hominids. Take courses in African wilderness survival, too. Get my eyes Lazicked so won’t need glasses
I’d kill any chance of hominids leading to Human evolution, by shooting, poisoning and introducing plagues that spread from group to group. History as we know it would cease in that timeline - Tabula Rasa.
sHOOTING HOMINIDS, MOSTLY.
No-one.
Guess I would lump an RPG grenade into the Wine cellar, where the first christians were holding their meeting and pick off the survivors with my sniper rifle.
(a Dragunov ok? plz advize)
My Messiach, I moof against dis Ork wit my zealot knife
I say unto thee hold thyself on, wherefore is the holy book of hits?
blam
What good would it do for me to go back into the past and try to change if, if I’m just the schmoo I am? I routinely fail at changing the present.
I’d go back to Mohammeds time and tell him God said you can drink beer
Because I’ve always felt that he was destined for greatness, and if he’d lived I’d want to know what he would do. Just a hunch, but it bothers me that I’ll never know.
Why not just do something simple – go back with a buttload of otherwise inexplicable “predictions” (sports scores, lottery results, freak events etc) for the next few weeks? Inlcude enough to them in the anonymous mailing to the FBI to prove that you have information from the future (make them memorable enough that whoever got stuck reading the “nut pile” that day would sit up and take notice).
Use one of the lottery numbers to get rich (several amazing wins by the same person could get you noticed).
Well, then let’s be fair. Make your historical change the death of JFK, arranging for him to be smothered between the thighs of a chubby hooker rather than publicly martryed. Without the legacy of Dealey Plaza and the salute and such, is JFK junior still destined for greatness?
OTOH, if you change the past of this timeline, then you might save some lives, but you are also preventing the birth of countless people who have been born since the POD (point-of-divergence – terminology from alternate-history SF) to the present. After all, each individual is the result of a lot of irreproducible rolls of the dies. First the parents have to meet, then have sex at the right moment, then produce one of – I forget how many, but it’s a very large number – of possible combinations of their respective gene-codes; and then the individual personality is formed by environmental factors even harder to reproduce, many of them imponderable. And IMO, getting your life cut short is better than never having existed at all. So there’s no sure way to do this without causing loss of life – unless you go with the OP’s scenario of creating a new timeline.
Great, except that it wouldn’t be possible even with the entire staff and computing power of the NSA in 2001. We don’t even have the technology today to simulate a dozen news anchors’ voices so perfectly that they would be indistinguishable from the real thing. A little digital analysis could prove that they were the real voices and not human imitators.
Well, it is a mundane, if impressive, technological feat – if you have a $350 million budget. Plus, you’d have to explain where all the actors got the time to film this and why no one ever saw them do it. Still, if you want more proof, how about if the time traveler brings back DVDs of every movie filmed in the world from 2002-2005. Could a huge bank of computers simulate the output of the world’s entire film industry for several years?
Then it’d be downright impossible for the news anchors in question to ever convince anyone they didn’t willingly and secretly participate in the making of the mock documentary. I mean, c’mon, I dislike conspiracy theories too, but if I have to choose between conspiracies and time-travelling aliens, I have to go with the mre plausible.
Besides, the exact computer capabilites of the NSA in 2001 wasn’t exactly public knowledge. I’ve no doubt all kinds of neat computer tricks are possible, incuding perfect simulations of humans, but the reason they aren’t (yet) common is that they’d be incredibly expensive and require massive (but not impossible) amounts of effort, and there isn’t a profit to be made.
True, but I still think you could fit a much more convincing proof onto a single disk of newspaper images from the weeks after your arrival. No computer simulation can predict next week’s exact sports scores, lottery numbers, stock prices, oddball news-filler events, etc, and the only plausible alternative (assuming you can prove that, yes, you did possess these predictions prior to the events) is every bit as implausible (a conspiracy big enough to make events work out the way they were “supposed” to in fine detail).
Go back to the Garden of Eden and beat the snake to death with a rock. Wouldn’t need anything after that.
I’d go back to 1967 and take out James Earl Ray and Sirhan Sirhan well before they killed MLK and RFK the following year.
Yeah, I’d do that too.
Or go back to 33 (or 29) AD and keep Jesus from getting killed. Then he’s just another rabbi with some pretty good ideas, no martyrdom, no Crusades, no Conquistadors, no Inquisition, no “God, King, and Country,” no Tammy Fae and Jim Baker, no Alan Keyes disowning his gay daughter. Maybe a little more tolerance. One would hope. 