My son lives up north and mentioned he wants to do some winter camping (only god knows why). I live in the south, and I know nothing about tent camping in seriously cold weather. For all winter campers here, what is the single, most advantageous product to buy for winter camping in your opinion? There are probably many, but what is the number one thing your happy to have bought? Are there several number ones? I want to surprise him with an excellent gift for this.