Back on topic, this is my favorite poster from the Fallout games:

I would pay some amount of money to have that on my wall, yeah. Or a t-shirt -- which is really just a poster for your body.
As for the time machine question, I think I would prefer to skip the grand gesture for my good deed in favor of a Quantum Leap-style "put right what once went wrong" kind of deal. But that assumes you've got someone clueing you in on the Divine Plan, and with my credentials that doesn't seem terribly likely.
For a fun thing, I would love to go back and see Led Zeppelin in concert. But there are many more Historically Important Things I'd like to see and do. For instance, I'd like to go back to around 1954 to see Sputnik in orbit and tell Joe McCarthy he's an asshole. I'd like to tour the South with the freedom riders in the summer of '61. I'd like to be in Dallas on November 22, 1963 to see if there really was anybody hanging out in the grassy knoll. (I wouldn't prevent the assassination, though, because of the butterfly theory -- step on a butterfly in 1922 and come home to a horrible dystopian society where everyone you love never existed.) So ideally I'd like to go back to sometime in the early 1950s and live through to the present day, but without ever getting any older.
Would I prevent 9/11? That's a harder question. Closer to home. Would it destroy the universe if I let the terrorists attack but made sure the buildings were evacuated first?