Toewarmers/handwarmers

It sounds like you are looking for the disposable chemical types. One friend purchased a large quantity of them for a very cold trip. After the trip started, he discovered that a high percentage of them had been compromised, and they wouldn't work at all.

I've found the reusable sodium acetate types to be good, but they will feel warm only if they are in an insulated space, such as at the foot of a sleeping bag, or within an insulated pocket. You recharge them by boiling them in a water pot for several minutes. On a winter trip, that isn't too hard.

–B.G.–

I tried one version of the acetate hand warmers maybe 5-6 years ago, and mine would only put out heat for a hour or so. I blew them off.

I like Grabber's various warmers. They make 10+ hour hand warmers, and just one is perfect for the foot of a sleeping bag (maybe place it in a sock). A pair weighs 1.55 oz. They also make a large 10+ hour "body warmer," which you could tape to a sore back muscle. I'll use one of these in my bag if I expect 0*F temperatures. Their toe warmers are made of a slightly different composition, they don't put out as much heat (don't need to for toes), and they work best in a low-oxygen environment such as inside shoes/boots.

REI sells the Grabber products, as do many other stores.

I've used the Grabbers for years, finally found a better brand at of all places a CVS drugstore. I'll have to check the brand, away from home for the week at the moment. The Grabbers are slow to activate for some reason.
Duane