Peanut butter, cheese, something with some nice fat content. Some good complex carbs too. And walk around a bit swinging your arms - a short, brisk stroll and a hot water Nalgene go a long way.... I usually try to change the minute we stop for the day, before temps go down, and either wash in the creek or take a wet wipe bath before putting on some nice clean clothes and warm socks for the night. (I'm frequently out where the temps are below freezing more often than not...) Hot tea/cocoa, hot broth, some hot food, gloves, and a good fleece hat/balaclava help a lot.