My first thoughts were "Kill myself! See what happens!", but now that I think about it that's really not too smart...
Kill Jesus... or... at very least the true founder of Christianity (I get the sudden feeling his name is "Bert"). If my calculations are correct, this will set in motion a chain of events that will drastically alter the modern day. Then I'd come back.
It's not that I want to alter the present in any particular way, I just want to do it for fun, see what happens.
Why Christians? Why not? I mean, I personally believe that all monotheistic religion exist only to cause suffering to others (I mean, you never hear of Hindus or Druids committing war crimes, do you?), I'd go after Judaism, because they pretty much started the whole thing, but there's no real time and place in their holy book for when it all took off, not one that can be easily converted into a modern timeframe, anyway.