Speaking of yeast, if you can't find that, you can use clostridium. I am not saying I'd actually use such a deadly bacteria, but people have over the years. And of course, make sure you bake the hell out of whatever.

Something that can be used for a lot of things is aluminum cans. Besides the obvious of using them to drink out of and store things in, there are many other ways to use them. You can build small stoves out of them, make a solar heater out of them, use them to make simple weapons, rig them as a makeshift alarm, or use them for fishing tackle.