Its been perishing cold, so I made the kids some wheat-bags yesterday.
I made an inner sleeve of calico and filled it up to about 3/4 with wheat and half a cup of lavender, then sewed the end shut.
Then the kids chose their favourite fabrics from my stash and I made a cover.
They go in the microwave for 2 - 3 minutes on high, with some water in a mug to stop the wheat drying out too much, and stay toasty warm for ages.
They can also be kept in the freezer, if you're after cold rather than hot.
I got the wheat from the local pet food store, and the lavender is leftovers from when Mumndad grew it out in Orange. I believe you can use rice instead of wheat, though I haven't tried it.