This is another easy project that makes nice gifts. Make sure your fabrics are natural to lessen the risk of melting or worse, fire in the microwave.
I like to make mine up as a little set, the rice bag itself, a washable sleeve to slip it into and a drawstring bag to keep it all in. This one is a nice purple flannel.
For those who have never used a rice bag, you warm the inner part in the microwave just like you do any other heat pack, then I slip it into the washable cover before use.
I've made tons of these over the years, in all sorts of prints and solids, enjoyed by older folks and children alike.