If your day hike is way off the beaten path and you get into trouble, you could easily be stuck out there overnight. For such hikes I recommend having a "backup kit" that will buy you 24 additional hours in the wilds. Something you can throw into your pack in addition to your usual gear, and then forget about if the hike goes as planned. My backup kit is shown above; it's a lidded pot, 5 inches (13 cm) in diameter and about the same height, packed with the items shown in the photo. The total weight is 27 ounces (770 grams) but I'm not so worried about weight; I'm sure you can come up with a lighter one. If you check out my first blog ever, you'll see a smaller, lighter backup kit.
As a day hiker you should already have enough clothing with you to make it through the night, if it comes to that. Still, I like the added security of a good quality survival blanket. As a non-smoker I keep my matches in my first aid kit, but the backup kit includes two additional ways to start a fire: a Bic mini lighter and a magnesium bar with flint. (No scraper/striker for the bar; if it's an emergency I'm not going to mind dulling my knife.) The round foil-wrapped items I'll explain below. The eight chlorine dioxide tablets are enough to purify eight liters of water. Once everything is in the pot, I tape the lid on with cloth surgical tape, as shown below. As I describe on a different page, the tape can be used as tinder.
Storing your backup stuff in a cook pot means that you can do two things. First, you can boil water to purify it. Second, you can provide something hot to to eat or drink. The dry soup in my kit has 360 calories, but if it's split among several people the main effect will be to warm people and buck up their courage as the long night begins. The tea (you could substitute instant coffee) will be a good way to start the next morning. Or on an awful night where people need to stay awake, you'll be glad for caffeine partway through.
The kit also includes four servings of peanut butter at 250 calories each, edible at any time—so a total of 1360 calories. In an emergency your main food reserve is body fat, but even a little additional food can help keep you from feeling exhausted and hopeless.
One of the foil-wrapped items is a Lightning Nugget brand fire starter. You put it under your tinder, round side down, and light the edge. The main reason I carry one of the size shown is that it
will burn a quarter-hour. When wood is damp it can take that long to get going. In contrast, magnesium scrapings will burn very hot for a few seconds, and then they're gone; they're a flame
source, not tinder.
The other foil-wrapped item is a tiny tea light in its own aluminum cup. You could also use this to help start a fire, either as the candle or as a source of wax. My main reason for including it is to use it as a lantern, with the foil wrapping shaped into a wind guard and reflector. For that reason the tea lamp is wrapped into a roughly 6 by 6 inch (15 by 15 cm) piece of aluminum foil.
P.S. Note the expiration date added to the peanut butter, since one isn't included on the individual serving. Eventually, nut products go rancid. Except for that, peanut butter is one of the great survival foods of all time.