I have had good luck with the AMK emergency bivy and thermo-lite bivy. They are durable and can stuff back into their pouch. The emergency bivy is small enough to fit in a cargo pocket. I have done some training for the boy scouts where I teach them to use some of the gear, and I have a AMK Heatsheets blanket which has taken a good bit of abuse, with no tears. It is also small enough to fit in a pocket.

I generally never travel without my poncho liner either, but it doesn't fit well in a pocket, so it is in my day pack. That and a poncho work surprisingly well. I have been in 20 degree Fahrenheit weather with only that, and didn't die. I wasn't very comfortable, but it worked. So if I am carrying a day pack or larger bag, I will have those as well. I also like the heavier Space blankets. I have used one of those with a wool blanket in very cold weather as well, and did well.

Another thing I like to have is a micro-fiber towel. They can absorb a great deal of water. I have used them to roll my wet clothes in and squeeze, to get them as dry as possible. Then I put them back on and wrap in my blanket or poncho liner to dry the rest of the way. It does make the drying and warming up process go much faster.