Water. Thats the big thing for being off grid. You can make your own water if you have a water source. There are many ways, but this is my method.
Couple of 5 gallon buckets, with a 1 gallon bucket or jug inside. A funnel. 1 gallon of plain bleach. 2-3 1 micron filter bags, like the kind used for salt water tropical fish tanks.
You can take pretty much any water there is, 1st pour it through a 1 micron filter. This will remove most impurities, and even some of the bugs that will make you sick. Dirty water will come out looking clear. Jug this water, or in to the 1 gallon bucket, and add 5 drops of bleach. Let it sit for a few hours. The bleach will kill everything in the water, but it is safe to drink at this low mix (I wouldn't go higher than 5 drops a gallon without first researching carefully) At this point, you will have drinkable water.
If you want to get the bleach smell and taste out, with a bit of plastic pipe, or a recycled can, build another filter: With fabric and/or fine mesh screen, make it so it securely holds activated charcol without it leaking out. Pour your water through the charcol, this will remove the bleach, you might have to do it twice to get it all. Again shopping around, activated charcol can be found pretty cheap. They say a teaspoon of it has more surface area than a tennis court, and just a few spoonfuls in a well made filter will filter out bleach for a lot of your water, certainly in the 1000s of gallons. I used 2 inch PVC to make my filter. Fashioned a a hanger for it with a coat hanger. Some old coat hangers in your bunker will come in handy for a lot of reasons. Or a big spool of bailing wire.
All of this stuff will fit inside a lidded 5 gallon bucket, and with a gallon of bleach you'll be able to make a couple of thousand gallons of drinking water. Look on eBay for the 1 micron filter bags, shop carefully, you can find them 2 or 3 for $20 give or take, not expensive. This whole water kit can be assembled for about $30-35, and will make water until you run out of bleach.
Bleach is also good, mixed 1:10 with water, as a sterilizing cleaner. So, storing 2-3 gallons of bleach might be a good idea.
I've got a small river right out back about 100 yards, and I keep this stuff on hand. I've tested this personally, and it works fine, the water is clean and has not made me sick. I would not hesitate to use any sort of river, stream, pond, or lake, even a farm pond. Water with chemical pollutants in it would be a problem though.
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The others have things well covered. Just remember stuff like vitamin C pills, calcium, and multivitamins. Every time you use a muscle in your body, your body uses calcium to do so, and without good sources of it in an off-grid diet, a calcium deficiency can happen within a few days. These things store well for years, never mind the "expiration" dates, if its within a few years it is perfectly good still. I keep a large bottle of tylenol, and with kids, I make sure I have this covered for them too. Nothing more miserable than a sick kid and nothing to give them to take the fever down.
If you have women around make sure their needs are going to be covered too.
