Interesting topic, JamieZ.
I divide up those dried meals (mac & cheese, hamburger helper, tuna helper) in half as most of the time that's about all I need. The packages keep well for several days provided you seal the contents from moisture.
I carry a few zip lock bags into every place I eat. If it's fast food, then I can replenish my salt, pepper, mayo, hot sauce, catsup, relish.... and if it's an all you can eat blue hair special I fill up the bags with goodies that'll last a day or so. Hey, it's all you can eat, but there's no mention of having to eat it there! And it helps if the jacket has big pockets.
For me, ramen style foods have lots of carbs, salt, and almost no nutritional value. Some days however, I find the need for salt as these taste particularly good.
I also cruise the food isles looking for the best buys as well as checking out the Managers Special section for meats. I carry a small folding ice cooler and with it packed with ice things'll stay cold for a day or so. At the soup bar I grab a dozen or so packets of crackers (regular or oyster), napkins, and utensils if they're available.
I carry a stove because I've found that I can cook a meal cheaper and faster than buying one. Some times I'm not in a location where finding food at a reasonable price is easy so I have a few things stashed away (tuna, sardines in mustard sauce, etc) that can tide me over.
Like many here, I like oatmeal for breakfast and throw in some homemade gorp to add a few more nutrients. The 12 pack assortment costs me about $3.00.
Don't be afraid to look in the discount cart at the supermarket. Some of these items are nearing expiration but if the can looks good (no bulging, dents, or obvious problems) chances are it'll cook just fine.
Don't forget signs in the window. I once got two huge pieces of peppereroni pizza for $.49 each by seeing an add in a storefront and found a small diner that served a 2-2-2 (2 eggs, 2 pancakes, 2 pieces of sausage/bacon, toast, and coffee) breakfast for $2.22, tax included. I also look at the local free paper to see if there are any ads for really special deals. You never know and if you don't look, you won't find them.
One of the things I like is 3-4 bean salad that's sold in the can. I'll grab a can, stash it in the freezer section, walk around a bit, and then retrieve it and pay at the counter. It gives me a cold meal that tastes better than store temperature and a respite from the heat. I've also done that with canned fruit, juices, etc. I also pass by the self-serve soda fountain and fill a zip lock baggie with ice for cool drinks later or to put in the Camelbak for later that day.
I've thought about the water quality but in the US it's pretty well regulated. It may taste like chlorine or worse but it is safe to drink. If you're really concerned then boiling, adding a small amount of bleach to the water, or adding one of those water purification pills usually does the trick. I've not gotten sick on water, but I have gotten sick on bad food or dirty cooking gear. And don't forget to wash your hands!
The NPS has a pass that for $80/year gives you free access to all the national lands. I've used it enough to more than justify renewing it. A look at freecampgrounds.com at any library will tell you what's free in your area.
I've heard mixed things about Wal-Mart camping. Evidently it's OK for RVs but not for tents. Camping nearby but not on the property probably won't cause a problem provided you're discrete.