I'd go play golf. After all, I would have access to the best courses in the world...might as well take advantage. The reason I would do this instead of something to help the country is that Congress still has the power and I am merely a figurehead that gets blamed for everything.

Of course is I were omnipotent for a day...

I am with Stagday, Valeogut and Grneyedblack...
Throwing my accounting background into the mix, I'd make the income tax 3 or 4 brackets straight regressive percentage...eliminate the loopholes. Of course, the lower tax brackets would be paying less than they are now which offsets the loss of deductions.