I wasn’t going to mention the Twin Towers (for the simple reason that it’s an important event in US history, but not as big to everyone else, and I belong to “everyone else”). But since you mentioned it:
I’d tell all NY Jews that there will be free food on the top of the Twin Towers on Sept 11th. Then persuade Bush that it would be a good idea to go talk to them, since they’re such an important community for the votes and so on.
It depends on whether you want to use the knowledge for good or evil. Are you trying to help people? And will they listen?
I could probably go back and tell my friends to invest in certain companies, or play certain numbers in the lottery. But would the entire population of Yugoslavia listen if I told them to evacuate the country before the 1999 bombings? Or would Madelaine Albright believe me if I told her that her favourite “freedom fighter” and his Taliban soldiers would be America’s worst enemy just 2 years later? And would she even care (because I seriously do not believe that Albright didn’t know who Bin Laden was when they were working together in Kosovo).
Ok, now I’m too political, but the question begged for it.
I’d probably use the knowledge to change my own life. Be more careful in my marriage (though it was this particular marriage that led to the particular daughter). Or invest all the money I made between 1998–2002. But I doubt warning anybody would have helped.