Crushed walnut shells are super to add to your pin cushions and give them some "heft". You can spread wool fleece or other stuffing materials around the edge of your pin cushion and pour the crushed walnut shells into the center. Your pin cushons will stay where you put them!
For a really FAT pin cushion, fill completely with the walnut shells for a satisfyingly heavy cushion. Critters do not like to eat the shells, humidity will not hurt them either.