OK, how about a cracky one. Harry and Fleur decide to go back through time and delete Riddle from existing, knowing that they will be stuck in the past due to the limitations of the ritual. So they appear in 1936, to take care of Riddle, which takes all but 5 minutes. However, for SOME reason, everyone else is convinced that they aren't Harry Potter and Fleur Delacour/Potter, but Winston and Clementine Churchill.
And while the obvious thing would be to take care of Hitler, you should remember that going full Morgenthau would mean that it will be Stalin's tanks rolling through Belgium in the 1940s, not German ones.