I use a packable duffel bag from REI that I picked up a few years ago for that very purpose. Its very small so its a no-brainer for me to toss it in my main bag on trips. As you can imagine a bag of that nature doesn't have a lot of structure to it so I'll often put my laundry in here an any pick-ups along the way into my main bag. That said it has been invaluable to me. Even used it in place of a daypack on a hike, slung over my shoulder like a messenger bag with passable results.

Not sure if REI still sells the model I have - seems to be listed as no longer available http://www.rei.com/product/748504. But this one is close: http://www.rei.com/product/778792

Looks like this Micro Duffel has a pretty mediocre strap though in contrast to mine (which is average but serviceable to start with). You can probably find others from eBags or elsewhere.