The TV show Seven Days addressed exactly this concept in each episode. The premise was that the government got hold of a small amount of alien fuel that allowed them to send a man back in time, but only seven days. In each episode the protagonist finds himself seven days in the past, with only his knowledge of future events and with all other people, naturally, not remembering a future they haven't yet experienced.
However, he had the advantage of working for a government supergroup with all the money and gadgets he could need; more importantly, they implanted a chip in his body that they had set up years earlier, so they could independently verify his claims. After the first episode, there was no worry that they'd disbelieve him.
If sent back on the morning of 9/11, I don't think there is any real way a single person could make a significant difference. It is possible, though unlikely, that you could get a plane ticket on one of the flights -- if you remembered the number -- and do your best to thwart the hijackers from the inside. Remember, rules were more lax, and you could bring all the liquid and shoes you wanted. On the other hand, that wouldn't guarantee any sort of success. The gun method mentioned in #2 would certainly have an effect, but again, it would only take one plane out of commission -- possibly two if they took off from the same terminal (see? I can't even remember if they did.) Personally, this would be my choice. With what I know about their methods (box-cutters, no guns) and what I know about self-defense, I think I could make serious headway in the narrow aisles of a plane and at least keep them from getting into the cockpit. I might die in the process, and I might accidentally injure some of the other passengers... but you know what, they died. Injury would be preferable.
Bomb threats would be very useful, but only if you could make them believable. Just calling in a threat would not be enough back then -- now, yes, but not before. You would have to know specifics about the buildings, their structure, the people on staff and how fast authorities could arrive. You'd have to be the most trustworthy bomb threat they'd ever received, and even then, the best you could hope for is shutting one terminal down and maybe, just maybe, evacuating the Towers earlier.
With so little time to waste, spending hours on the phone might not be the best option. You could steal a taxi and ram it into the front gates of the Towers, or try to run security at the Pentagon; unlikely to cause a major evacuation, but it would get emergency help there faster, and they would be in a better position for the inevitable attacks.
#s 5 & 6 are absolutely correct; imagine phoning 911 with all the details you can remember, only to have the recorded tape used as testimony at your trial. No one would believe you were anything other than a part of the plan; even if you could convince the government that you were from the future, they'd keep you locked up forever anyway. And they'd make you invent the iPod, iPad, Kindle, Wii, and SD card for them. (Can you imagine? "I don't know, it's a little chip of plastic, the size of my thumbnail! It holds eight gigabytes of information! I know computers only hold five hundred megabytes!"
With all that said, I cannot imagine anyone in that position simply giving up and living through it all over again. I'd hope, for the sake of humanity, that the unlucky time traveler would have the nerve and the courage to stand up and be thought a lunatic or worse. To try and fail is better by far than to not try at all.