I second external battery packs. Lots of them available already, as previous links point!

I wouldn't use AA batteries, but rechargeable AA cells instead... Think of the environment! Put 4 or 5 of them (4.8V or 6.0V - someone prove me wrong, please!) together, add expensive, top-quality regulator, USB cable, some casing and you're done. First I laughed at chargeable chargers - out lout - but if they pack enough power, they just might save your week...

How about solar cells? Limited use cases, but hey, free power! Also widely available...