Back in the days of my youth when campfires were routinely used for cooking and warmth (1940s-1950s), a hot rock inserted in the sleeping bag was a great foot warmer. This method had its problems, though, particularly if there turned out to be moisture inside the rock while it was being heated in the fire, revealing the explosive potential of steam. That could get rather exciting!

Like W_D, I won't put a water bottle in my sleeping bag. Now that I no longer have a dog, I've discovered down booties. Chemical warmers work, too, and stay warm much longer than the hot water bottle or rock. Finally, wearing a warm hat can be a surprising cure for cold feet, as I once again realized just last night (we had record low temps in the Portland area last night, and my feet got chilled before I went to bed). "When your feet are cold, put on a hat" really does work!