If I could, I'd fly around Earth in the opposite direction of its natural orbit, fast enough and long enough to cause it to begin spinning in this opposite direction. Obviously, at this point, time would also reverse (naturally), and I would be travelling back in time. I would stop off on the morning of the horse riding accident and deflate his car's tires, slip his horse a segagive, yes, segagive (see Young Frankenstein), cut the power to his house so his alarm clock doesn't go off, do anything to prevent his tragic destiny.
If you've not seen this movie recently (I know A & E has shown it a couple of times this summer), you ought to rent it or watch it again, somehow. It's a classic. This is one of John Williams's best films as well. Many of the songs have the great aching, romantic sweep of things he wrote for The Empire Strikes Back and then Raiders of the Lost Ark.
The new Superman flick (Superman Returns), due summer 2006, looks promising, and supposedly was made as an homage/"sequel" to Superman: The Movie. The guy playing Clark Kent/Superman even looks a little bit like a young Chris Reeve.