Most of my business travel has involved situations where field work was on the agenda, so I was in good shape for a good many potential emergencies in terms of FAKs, etc. I found that a small, minimalist backpack of the type that do double duty as a stuff sack for a sleeping bag made for a very versatile carry on bag that was quite useful in a variety of situations. I always try to have shoes or boot with me in which I can walk a significant distance, if necessary. This can be tricky if your work involves more formal environments.

I think it is hard to bear Chaosmagnet's basic list.