Afll of the above - also, after spending a long 12 hours in Iqaluit airport a few years back without a hotel (think Nunavut, way northern Canada, Baffin Bay etc), I now pack a sleeping bag liner in my carryon. And if I have room, I also pack a sleeping pad, I have an inflatable Big Agnes that packs small but blows up and is really comfy to sleep on. The sleeping bag liner I think is way more comfortable to sleep in than throwing your jacket over your shoulders. They come in silk (popular), fleece, flannel, and cotton, and run maybe $30-60 before sales - http://www.rei.com/search?search=sleeping+bag+liners&seq=1&hist=query%2Csleeping+bag+liners.

I'm partial to a little food to get me by in airports - pack a few power bars, they're better than nothing. Also some hard candy - that's easy to spread around, especially with kids who can't tolerate flight delays nearly as well as most adults.

Also my Zune music / video player - loaded with music I like to listen to, I also load a few movies or TV shows before trips so I can watch them. Also sorta cool is Comcast's Infinity service, where you get your cable TV content over the Internet wherever you are - if I'm in an airport waiting for a flight or even at destination, its easier to fire up the laptop to watch a movie on HBO (or Netflix, don't forget that) rather than pay hotel rates for access. And I also keep my 3G/4G adapter handy, with it I can get Internet access from anywhere that Sprint has coverage. I like travel with a full laptop more than a smartphone, but you can also get most of the same stuff, just on a smaller screen.

FWIW I've never gotten into carrying spare clothing in my carryon - I tend to wear a Patagonia capilene base layer, maybe a wool sweater or shirt over that, and a fleece, or warmer jacket if I'm headed to a colder destination. The capilene is good for at least a week before it gets too smelly or troublesome - and if I need a shirt change, I don't care too much what it is, I'll pay $20-25 for a souvenir t-shirt at a news stand. Also I carry gloves and a warm fleece hat most destinations - if I leave an airport, and have to head home overland, its one less thing to worry about. Also I wear wool slacks most flights, not levis - that just comes from having to check in when you land, I'm convinced you get treated just a touch better if you dress a bit better. ymmv