I made rice bags with removable, washable covers last year for Christmas presents. I use muslin for the basic bag and made covers out of flannel or fleece. They were well received by those who got them.
I heat one in the evening as a nice gesture for my DH when he's had a stressful day. Besides aching shoulders and neck from sewing or computer, I use them across my sinuses to relieve colds and headaches or wrap around my feet on a bitter cold day if my feet feel frozen. I even keep one in my desk at work to use on days when I have to huddle for hours on the computer to meet a deadline. That and a stretch while it is heating in the microwave are very helpful.