Do some Googling for "Freezer Bag Cooking" and you will find http://www.trailcooking.com/ among others. The "Cozy" is key to keeping the warmth in while the food "cooks", but a fleece hat inside an upside-down bike helmet would do pretty well.

Instant mashed potatoes with a swirl of peanut butter may taste good, too.

Many of the recipes you'll find are a little involved, but with ramen noodles (look at the packages as a source of ingredients instead of the only thing you'll be eating), it's pretty easy to add a variety of other flavors. Pick up a can of tuna to stir in for tuna ramen, etc.