Crazy answer: The moon to 1969. Imagine, Apollo 11 launches to land on the moon then suddenly a second one appears in space orbiting opposite the original.
Less crazy but still far reaching: a tazer to the theater the night of the Lincoln assassination. I wonder how different the U.S. would be if Lincoln had survived and the resonstruction period gone differently.
Somewhat more plasuable: enough documentation and evidence to convince someone high up in U.S. intelligence in 2000 that I am a time traveler then everything we have on the world trade center attack. I'm curious how different things would be if that had failed. Maybe bush only gets one term and the whole U.S. political situation shifts for the better. Maybe a less destabilized middle east avoids the worst of ISIS and with fewer syrian refugees, right wingers in Europe are less successful, Brexit fails, and Russia declines to invade Ukraine.