What a great question! I would end all wars, feed those who needed fed, allocate research funds to our most dreaded diseases, make laws limiting big business, many other things. It might take more than one day to do all this!

I agree that one day wouldn't be enough. You would have to fundamentally change human nature in order to really remedy many of the problems facing the human race. You would also have to fix some of the basic structual issues involving scarcity of resources and imbalance of wealth. It may sound a bit pessimistic, but I can't imagine anything I could do in a day, even given almost omnipotent powers, that would make much of a difference.

I agree that one day is not really long enough to make a difference.  The only think I can think of is to take a Michael Corleone approach and kill all of the leaders that I did not like.  For instance, killing warlords and drug dealers and dangerous dictators might lead to more stability in the long run.  I do not know if one day is enough time to get that kind of manpower in place though.  It would have a large effect, but I am not like kind of person that goes around killing.  I don’t know how it would affect me, especially if there is collateral damage and innocent people get hurt.

I love this question.  My initial, self-serving thought, would be to name myself ruler of the planet.  Ego aside, if I could do one and only one thing to change the world I would throw every resource possible at creating a machine that would desalinate water while creating energy.  This would solve our planet's destructive addiction to oil and provide clean drinking water to everyone.

We can intepret this question as asking: If given the power to change anything for a day on a global scale, what would you change? (Given this intepretation, I'd make sure that everyone everywhere got to eat three meals on the day I was in charge.)

We could also intepret the question as asking: If you could emphasize or bring focus to one thing on a global scale, what would you choose to emphasize? (For this version, I think I would organize concerts in every city and possibly turn off the television and internet service in every city for the duration of the concert in an effort to communicate the tranformative power of art and music in our lives.)

I don't think that I'd be able to do anything that would matter.  I could, for example, order human rights to be obeyed everywhere.  But that would not last past my one day in power.  None of the things that are important problems in the world can be solved by the actions of one day (sadly). 

I suppose if I had to pick what I'd do, I'd impose a two state solution on Israel and the Palestinians.  I'd force the Israeli settlers out of the West Bank and I'd force the Palestinians to accept Jerusalem as an Israeli city.  

But again, that wouldn't last...