I've used home made wheat and rice filled ones. They did smell different (though both were fine) and I think the wheat held the heat for a bit longer.
I've only made rice ones though as I don't have anywhere close by to buy wheat economically.
I have a couple socks filled with barley. They hold heat well and I personally love the smell - similar to brown rice but a little different.
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I've used oatmeal in some to make it a lighter pillow like feel. Doesn't hold the heat too well but that made it perfect for a little one.
I like to use brown rice as the larger pieces seem to hold the heat better than white rice.
I've read that corn kernels work great (popcorn).
I've added dried herbs in a few times but they don't smell as strongly as the heated rice so it seems a waste to me!