I was recently lucky enough to try Kushyfoot Flats to Go, which are designed for travelers as well as anyone who needs a break from their heels. They are ultra portable, and can fit in nicely into your laptop bag or medium to large size purse.
These are for indoor use, and could possibly work outdoor in dry weather. They are really thin, so I wouldn't rely on them to keep your feet warm, just comfortable.
They slip on like a glove and provide relief from heels or uncomfortable shoes. I usually wear these into the airport, through security, on the plane, and to the rental car station, before I even put my heels on, saving my feet hours of torture! I love how they fold right in half and fit nicely into their clear plastic pouch they came in.
I tested out the snake print flats. These have black soles and a little snake print bows on top. I normally wear a 6.5 size shoe, and size 5-6 Kushyfoot Flats to Go fit me great. These are way to casual and thin to pass as "normal" shoes, but you will definitely see me wearing these as long as possible before I need to put my heels on. They retail for $9.99 and can be purchased from Kushyfoot.com. I would definitely recommend keeping a pair in your work laptop bag, purse or car, for emergency heel situations!