You are right that we should never take ourselves too seriously, so in that frame of mind I offer the following (unfortunately mostly bad) 9/11 jokes:

Did you hear the one about American Airlines new deal? They’ll fly you straight from the airport to the office.

Knock knock!
Who's there?
Knock knock!!
Knock knock who?
Knock knock knocked all your towers down!


What is the New York City Fire Department's favorite song?
"It's Raining Men"


What's Al Qaida's favorite football team?
The New York Jets

What does WTC stand for?
What Trade Centre?

When does a pentagon have four sides?
When it intersects a plane.

What was the last thing going through Mr. Jones' head when he was working on the World Trade Center's 90th floor?
The 91st floor.

What's the last thing that went through the minds of the 9/11 jumpers?
Their ankles.