What a terrific topic! Wish I had discovered it a year ago. I travel mostly for pleasure and have never mastered the art of carry on only that so many here have. What I do tho (in case I am separated from my luggage) is have every essential for survival in my rolling lap top case. I can get by for a couple of days with the toiletries stuffed in a Delta ammenities case, extra socks, underwear, shirt, etc.
I have learned each time I was stranded a little more about what I really need in this event.

I always take water and a snack and insist that my kids do so in their backpacks. You never know when you need that snack and you can always toss it.

I swear by ziplock bags. I even double bag things that could spill. I put nail polish remover on cotton balls and put those in their own ziploc.

I use a packing list that I found in Fodor's book: How to Pack. I keep one in my travel bag with essentials that never get unpacked at home. I seldom ever need to buy anything that I forgot to pack.

I bring a small framed photo of my children if I am traveling alone to brighten up my hotel room.

Like someone said above a small umbrella is a must. Mine gave me plenty of shade in Turkey, Greece and Egypt this summer. On that trip I used for the first time a toiletries kit like the one Punki mentioned in her first post. It hangs over the door and opens out after unzipping (protects against spills enroute). I love it and will never unpack it, only refill now and then.

Also helpful is a small spray can of Lysol to degerm the hotel bathroom and give me peace of mind. Maybe I need to have my head examined on that one - a bit extreme, I know. A small roll of duct tape often comes in handy.

And lastly, at the Container Store I bought heavy duty plastic "suck down" bags. I can cram them full of stuff (like ski clothes or dirty clothes) then borrow housekeeping's vacuum and voila - the stuff takes less than half the room.

Whew! Sounds like I don't travel light at all! I do try to tho!