Well, I spent the entire day remembering 9/11 by flying on commercial airlines through 3 different airports. I left at 7 am and just got home. It was clear that the police and TSA presence was markedly increased with extra screening and pat-downs...but everyone, including the airlines, was being low-key and careful to avoid drawing attention to it.
They were all being extra nice to the passengers, but even with the extra kindness it kinda hard to not notice that there were 6 TSA agents surrounding every passenger after they came through the body scanner and got a pat-down. About 1/3 of all bags I saw going through security were sent through the X-ray machine twice just to double check. There were also extra fligh attendants on each flight. Planes than normally had 3 attendants had 5 today.
When I flew out on Monday through the same airports and on the same trip legs, it wasn’t anything like it was today.