Dagny mentioned the booster cables, I will mention the tow strap.

Did anybody suggest some munchies in case they are stuck for a few hours?

LeeG mentioned the gloves, but I will repeat that anyhow. It can be real hard to change a tire or shovel out with fingers that are freezing cold. Crappy looking mitts are warmer than gloves, one size (Xlarge) fits all and they are less likely to go missing from the kit.