...what would you do if you woke up on September 01, 2001?
This is a more compelling question. Although it leads me back to the topic of the thread
First I'd kidnap myself so that future me is the one arrested/blamed for the mostly illegal crap I'd pull...
Going around preaching about the future or trying to get people to believe you are a time traveller is pointless since everyone would just assume you're insane. The only way for a normal individual (non-high ranking politician/world leader) to stop 9/11 would be to do one of the following (at least as I see it):
1) Call in a number of repeated threats to various government agencies or the WTC to try to heighten enforcement or evacuate the WTC.
2) Highjack/blow up a small plane in order to increase airline security.
3) Fly a small plane into the WTC to show what could be done to raise flags and tighten security.
4) Find the hijackers and take them out before they can get on the planes.
5) Some combination of the above.
I do realize that some of the options have some innocent casualties as well but compared to the death toll of 9/11 a few may have to be sacrificed. At any rate I'd make sure that my past self was locked away somewhere safe, since I'd be on the fast track to getting myself arrested or killed.
Now if I woke up as my past self and was the only "me" present then things get trickier. I got married in 2002 and had kids in 2004 and 2007. I know that the fate of 3000 people should outweigh my little family in the grand scheme of things but anyone who says that would be an easy choice doesn't have kids yet... I could sacrifice myself, knowing that the "past me" would be around to start my family and I might be able to sacrifice others if the need arose, however, the line is different when the sacrifice are my kids/family as illogical as that may be.
If I had to choose someone to send back, the above still stands so, ideally I'd send back someone high up in the FBI/CIA that could get the hijackers arrested early enough to stop it.
If we exclude high ranking government officials then I'd probably choose someone who knew how to fly small aircraft who had little or no family. They'd be sent with a list of people who worked on the upper floors of the WTC to call/try to keep from going to work late on 9/8 to minimize casualties. If no other method appeared to be working they'd be under instructions to call in a threat and fly a small plane into an upper story of one of the towers on 9/8, preferably late at night to avoid as many workers as possible. As I see it, a normal individual without government access/powers, would probably be forced to pull some sort of an attack to heighten awareness and security and would have to be very publicly outed as non-middle eastern and Christian (video confessions send to the news outlets and plenty of witnesses that see them get on the plane) could put us on a path of increased security without the xenophobia that led us into the Iraq quagmire. Most of this reasoning is due to the short timeline imposed, if we could have the person sent back years in advance we could use him to "tip off" the authorities on other attacks to establish credibility and have him point to the hijackers earlier to get them arrested/prevented from boarding on 9/11 or possibly just to infiltrate the hijackers to feed intel to the FBI/CIA.
Of course this does not do much to thwart possible contingency attacks from other cells that might have been in place had 9/11 failed but I doubt that any of those could have been as devastating as 9/11. If they had access to nuclear or radioactive material for a nuke/dirty bomb I would think they would have done that rather than the planes or at the same time since given the fear surrounding radiation/nuclear attacks they could have caused more of a disruption/more fear through the bombs than the planes.