I like to take advantage of long layovers to see a little bit of whatever area I'm in, or at least explore the airport a bit rather than just sit at the gate/lounge. I would find it useful to have threads of things you can reasonably do from an airport with a few hour layover. I'll start with some ideas from Dulles since that's where I'm based (though that means I've never HAD a layover at IAD so others may know more) Please add more ideas! Would love to see these for other cities (a la the Whirlwind column in Hemispheres). There's always the Udvar-Hazy Center to see lots of different aircraft (as the mobile lounge audio track will tell you). Ride the mobile lounges at 5pm and see lots of planes heading out in the 5pm bank. If you're into history, there are some nearby Civil War battlefields and the Sully Plantation.