What are some cheap ways to create heat without electricity or toxic fumes?

I ask for camping. I wonder if there are things more price-efficient than hand warmers for heating the body.

Or are there ways to chemically heat water, cheaper than self-heating food packages. (5-10 dollars per pack)

(No propane/buthan or alcohol burners because of small air volume. Something that can be mixed in a pack like a hand warmer.)

Any good ideas?