Rice is probably your best solution because:
1. It's probably the cheapest
2. If you're ever lost in the wilderness, you can eat it.
3. It doubles as an excellent moisture absorber, especially the "instant" variety
Don't worry about spills and the old caution of rice killing birds if they injest it -- that's purely an urban myth and no fact to it at all. Bird seed isn't a bad idea, but I've yet to buy a bag of bird seed that didn't end up having little bugs hatch in it eventually. Bird seed is also typically dusty (unlike rice), and you probably wouldn't want something dusty in yourgadget bag.
I made my own little pillow caseout of scrap material, and then I put rice in a ziplock baggie and put the pillow case over it. I also have a velour-like draw string bag that came with...something...(can't remember what) and I will also use it a as pillow case and even an extra protection type thing for my camera (when the bag of rice isn't in it).