I don't think anyone has mentioned Tasty Bites yet. These are my favorite "MREs" that can be bought at the supermarket. They are vegetarian or vegan, have a really short ingredient list (real foods), and are in a sealed plastic bag that you can put in boiling water. I've put them in hot spring sources to heat up. They are even OK cold, when you're in a hurry and don't have time to heat them up. I've even tossed them on top of the car's dashboard in the sun, and they get pretty hot that way, too. The only problem might be if you don't like Indian/Asian cuisine, which is what they are. They are between three and four US dollars each - a little pricey but damn convenient. Hope this post doesn't sound like an ad, but here's their site:
http://tastybite.com/products/ 