Hand warmers or boot warmer packs, held with a couple of large $1 store, elastic bands. Can be helpful to keep battery packs warm too..
Oh by the way, those packs will stay warm for some lengths of time.
Foot Warm-Up Foot Warmers packs are good for up to 6 full hours.
Grabber warmers Toe Warmers 6+ hours.
Johnston HotShots Hand Warmers up to 3 hours of warmth.
So you all better bring some snowshoes or ice skates too. ;-)
And a large thermos of cocoa... Say Ahhhhh!! Yes, that's hot.
RedHotPoker