Indeed many alternative media I've used or seen used. I've tried:
rice, groats (wheat, oat, or rye), pine needle (loose or bundled),
lentils. soy beans, other dry beans, well rusted & wire brushed steel
plates, dry rocks with interesting textures, weathered wood (end and
side grain), that's all I recall...
I've heard, seen or considered: any shape of pasta, corn broom
heads, field corn, dried corn cobs, tree bark, straw, pine cones, or
most any organic material. For a slightly more planned casting,shape
pottery clay with any fairly shallow texture or image, with no under
cutting or you'll wind up with clay inclusions.
Now, most importantly, HAVE FUN!!