Stumbled upon this novel idea. Not sure how good it will work but Im going to try it and at the very least it will go in my oddball playing card cabinet.
http://news.distractify.com/beth-buczynski/how-to-recharge-batteries-with-just-a-playing-card-and-the-sun/
Actually, it should work fine, if you wire it up correctly. The playing card is really incidental - it could be mounted to nearly any firm-enough surface. Imagine placing those solar cells all over the back of a cereal box or a pizza box - you could charge a plethora of batteries at once with a single device still small enough to be easily transported.