Being omnipotent is most likely every person's fantasy. Personally, I fantasize about having supernatural power for its own sake. Having a time limit on it is a safe idea because it can serve as a test of character to see if it will bring out the best or worst in you.
As with most people, I would accept such an offer almost immediately provided there were no strings attached. However, one of the first things I would do that I doubt others would do would be to aid humanity. I wouldn't simply fix all of their problems, because they would never learn that way. I would help them to understand what they are doing to each other and I would help to bring a limited peace to the world in that one month. Granted, I would also make sure my family and friends were set up quite nicely before the end and I would probably give myself a way to regain some of those powers after the month has ended, but no one is perfect.
Because then maybe we could do more than just scratch the paint off their cars. True maybe with unlimited power I would become just as corrupt as they are, but then again my power would be limited to 1 month (well now, there's an idea). That would mean I would have just a month to right some of the many, many wrongs put forth by those who pretty much have unlimited power 356 days a year, year after year. Making the end result a better place to live in for not just me but the majority of people for when my power runs out.
I even know what exactly I would do with that much power though I don't even need that length of time so I am unsure what would occur after the preliminary events I think I'd be at odds with myself for the next 25 days or so. I guess maybe I'd just become omniscient or something and grant myself various secondary sources of power.
I'm one of those people that wants to know everything. So I'd use my powers to grant myself ultimate knowledge. I imagine that knowing everything would make me very nihilistic, and I might end up abusing the power I have or simply wasting it, when I could be curing human suffering.