Potato guns are very versatile firearms. They can fire almost anything. If something is too small to fit in the barrel, you just put a potato in first or wrap it with a paper towel. If you run out of stuff to shoot, just pick up dirt off the ground, get it?
Potatos-obviously, hence the name. Potatos are very good things to shoot if you can get them to seal correctly. You just cut it so that its flat, and push it in carefully.
Citrus-The very best organic projectile. Take your barrel, or a piece of PVC the same size to the store (I'm serious!) and look for lemons or limes that fit it. If you can find on that fits perfectly, it will shoot amazingly. Just find ones that fit fairly tightly, yet the skin remains intact. When you put it into the gun, you may want to lube the barrel with WD-40. Citrus is great cause it also explodes violently spraying its juice everywhere, and won't break anything (don't take my word for it, read the disclaimer first). I love lemons.
Steel-I recently made a steel round, it actually works pretty well. Its just 3 washers, slightly smaller than the barrel, that are on a bolt. Take your barrel to the harware store and find some washers that slide freely in the barrel, then get a bolt to put them on, and some nuts to attach them to the bolt with. It should be a hex head bolt, and you should have the head of the bolt, then two washers, then two nuts, then another washers, and two nuts to finish it off. The bolt shoudn't extend too far beyond the last washer, so get one that isn't too long. It looks like this --|--||o the dashes "-" are the nuts, the "|" are the washers and the "o" is the head of the bolt. This is a great round, b/c it is reusable, but be VERY careful as this could probably screw something up really bad.
Beans-This is very interesting. Beans act like buckshot, as the spread when they exit the barrel. There are two ways to make a bean shot, you can make one that explodes on impact, or one like a shotgun that just exits the barrel as many particles. To make the one that explodes on impact, take half a paper towel and pour about 20-30 smallish beans in the middle. Wrap it up and fold it over, put the folded over part into the barrel first. To make buckshot, don't fold it over and just gather it up and put the excess of the paper towel in last.
I can't think of any other ammo, but if you think of something, mail me and I'll add it to the list.