I would have a conversation with everyone in the world for the day. Talk to them about how they're living their lives, guide them on a better path were they think more for themselves, and less like lemmings who will willingly jump off a cliff shouting 'yippee!'.
Not only would this give me a better perspective on the world, but I would also be able to get people to ask themselves some hard questions, and also discover a whole bunch of wacky secrets.
Then I would go back in time I get rid of all incredibly shit music, starting with Katy Perry, and working backwards from there.
And then I would make 'them' make all the awesome stuff they should have already made, such as more seasons of Firefly, an Alien vs. Predator movie that doesn't suck, so on and so on.
I would also give myself a perfect clean bill of health, so that I could die of old age at 100+. And lots of money.
And if, and there's a big if, there is such a thing as true love, I would like to know who that is.
I wouldn't want to be omnipotent forever, but I guess I would try and find a way to hold onto it for a while longer. Power corrupts, I've read enough comics to have learnt that lesson already.
Oh, and then I would go back in time, and snatch key figures out of history just before they die, replace them with a soulless clone then have lots of fun. Imagine Jesus vs. Hitler for real? Or Ghandi vs. Martin Luther King - the awesomeness!!!