I never go anywhere without at least a pair of LEVI'S as you never know when the weather will change (or you get stuck in a connecting city). You should also bring GLASSES as it is possible that you will experience eye irritation (due to smoke, etc.) and you won't be able to wear contacts. Ditto for SUNGLASSES, though not an expensive pair as they sometimes disappear. In addition, I would carry-on one long-sleeve shirt or sweater, as airplanes are often drafty. An old baseball HAT might also prove useful, especially during a brief t-storm.

If you are checking luggage, I would also throw in a SWISS ARMY KNIFE which I have found useful in many ways.

I concur with the disposable clothes recommendation, particularly for old t-shirts, socks, etc. No reason to carry that stuff around, and you can always buy more if for some reason you run out.