The new library -- which must border on Orwellian surrealism -- asks that question?
Okay, a few things I wouldn't have done:
I wouldn't have sat in classroom for 30 minutes after I was told the nation was under attack, and would have wanted to know why my secret service detail didn't hustle me out of there in violation of all protoco.... hey, wait a minute, then he and they must have known something. What I would have done is cancelled the war games drill and false flag operation the way JFK knocked down Operation Northwoods (which horrified him, btw. I believe it was at that point that he finally realized the insidious evil he was dealing with). And I wouldn't have brought in a whole crowd of neo-con Israeli-first crazies to run foreign policy.
I wouldn't have invaded Afghanistan, droned Pakistan, or told lies about weapons-of-mass-destruction (the same lies the AmeriKan ma$$ media is now spewing about Syria and Iran) in Iraq. 'nuff said.
I wouldn't have tortured innocent people that I knew were innocent. And I would have let go the people that are being held in perpetuity. I would have called out all the lies that led to torture. I wouldn't have used chemical weapons in Iraq and Afghanistan, and I wouldn't have allowed Israel to get away with their occasional forays and never-ending siege of Gaza. Of course, by that point I would have been dead somehow.
I wouldn't have allowed the Wall Street banks and other corporations to run roughshod over the economy and develop their fancy looting schemes as they did during your tenure. I wouldn't have allowed the fraudulent foreclosing of homes, although we are now straying into the area of your successor. You just left him the mess. I wouldn't have bailed them out, but at least some Enron crooks went to jail during your term. Obama considers the banks too big to jail, and is thinking of letting Skilling of Enron out early.
I'm sure if I sat here and kept typing I could think of many other things, but those above are a few of the larger items I would have done different.