I carry a few of these when I tramp around. They are great for a cup of drink or a warm meal and can be bought cheap. Calcium Carbide has numerous other uses besides cooking but i'll leave that to others. Here's a link to bulk heaters. Nitro-Pak: MRE Heater Paks - 12 Paks of 12 Heaters - MRE Heaters - MRE's - Products
Make your own heaters with Citric acid and powdered iron ( filings), just add water and presto! Heat ( and steam!)
What are the proportions of Citric acid, iron filings, and water to be used for the homemade version?