re: hitler:
http://www.abyssapexzine.com/wikihistory/i'd try to prevent 9/11 by going back to 1998 and inventing a scandal that prevents katharine harris from being elected as secretary of state for florida. snuff porn in her laptop or something puts a democrat in that seat, altering the catastrophic 2000 bush/gore election results process by enough to likely give the seat to gore, who wouldn't have been on vacation four months of the first eight in office. if gore could have prevented 9/11, he would have, and if not, we probably wouldn't have gone into iraq again, destroying the global goodwill toward the states; we might still be in afghanistan and we might not have elected our first black president, but hell who knows? the republican party might have completely imploded by now, and we might remain a bit more centrist a nation, rather than getting caught up in this ridiculous constant rightward overton window shift.
after which i'd go to woodstock, take some weed from there back to louis armstrong's youth and hang out with him in new orleans for a week, steal and stash his original coronet (and gift him a new trumpet), which i'd "unearth" a couple weeks before he dies and return it to him face to face, just to give him a "what in the how" moment.
then it might be fun to just skip forward a year or five once a day or five, to see how long it takes us to reach certain achievements: peace in africa, widespread adoption of wearable/embedded electronics, interplanetary travel, first contact, that kind of thing.