There's a name for the survival sack that you take with you outdoors in the event of survival emergency: bug-out bag.
Let's start with the key components to net a three-day survival:
- Water: one liter per person per day
- Food: "energy bars" or backpack meals
- Small pot or large cup (though if you have only energy bars plus iodine tablets, you won't need to boil water for food or purification).
- Clothes: sturdy footwear, long pants, two pairs non-cotton socks, two shirts, rainwear hooded jacket and rainwear pants, long underwear, wide-brimmed hat
- Tarp or tent plus a ground tarp; sleeping bag
- First aid kit (not necessarily a prepackaged one from the store; it may be better to build one; you'll know exactly what's in there, like tweezers to remove ticks).
- Fire starters
- Survival knife (find the one that suits you best)
- Small mirror (in case something gets in your eye, but also to reflect the sun to get the attention of rescue aircraft)
- Two flashlights and backup batteries
- Weapon (the knife may suffice, but you probably won't be too confident with only a knife to fend off a bear, so better have pepper spray on hand) If you are a gun person, please be properly trained.
- Baby wipes. Hygiene is as important as nutrition.
- Sunglasses and sunscreen (imagine the sunburn three days out in the sun, even during winter; snow reflects sun from a clear sky like mad).
- GPS or some kind of beacon to help find you if you get lost.
- Paracord. Google it.
There are so many more things that can be added to the bug-out bag, but remember, this list refers to three days' worth of survival. Obviously, if you want to pack the bag for seven days, you'd want to include more things. These additional items may be anything from a map and compass to a snake-bite kit.
Small plastic bags and long shoelaces are also invaluable, as they can be used to trap water moisture from non-poisonous vegetation branches and condense it over several hours, filling the bag with enough to drink from.