Too bad you couldn’t get them to come to blows so much that they kill each other.
Posted earlier, but upon rereading the OP it said four things you’d change. So I figure I have a few more coming:
See if I could get the Viking colonies in N. America to not die out. For example, have the Vikings accidently introduce smallpox to the New World. The native population takes a terrible hit, but it had to happen sooner or later, and when the native population rebounds they’re now composed of relatively immune survivors.
The respite gives the Vikes time to establish a foothold on the Atlantic, but without firearms they can’t overrun the whole continent. What they do do is introduce cattle, pigs, horses and ironworking to the natives, and establish a coastal trade as far south as the Aztek empire and the Carribbean. The improved food sources and technology vastly strengthen the native cultures.
So when Europeans begin crossing the Atlantic with transoceanic ships and guns beginning in the fifteenth century, they find an America where nominally Christian whites already hold the northeast; and the rest of the hemisphere is relatively “innoculated” against European invasion. The murderous Conquistadores would still have eventually conquered the New World, but the native cultures wouldn’t be utterly wiped out.
If I travelled back in time, I highly doubt I’d get a private audience to influence people like political or religious leaders.
However, it would be fun to give Judas 31 pieces of silver not to report Jesus. Jesus doesn’t get his martyr’s death and see what happens.
But I’d do sillier stuff like that poster who put Star Wars figures in a future archaeological digs.
Or go where I know there will be a UFO sighting, and take a non-blurry photo with a wide-angle lens or unequivocal video footage with an advanced camera. And, most likely, expose the hoax/mistake.
If I were to do a sports bet, I would create the perfect NCAA bracket. Every upset, every game perfect. Enter it in all those public contests like at Vegas sports books, ESPN.com, sandbox.com. Not so much for the money, but to have the sporting world scratch their heads how I knew Nevada and Alabama-Birmingham would be in the Sweet 16. I would be a legend.
Probably your time machine was the reason for the UFO report 
Finds it very interesting that so many dopers have it out for Religous founders
I’d personally work on a few popes instead, to nudge the Christainty in the right direction.