For a one day excursion, what would be a must-have in your backpack?

First aid kit.
WATER.
thermal blanket
water proof matches, or a lighter.
a few granloa bars.
also make sure your cell phone is fully charged, and tell someone where your going and roughly what time you will be back.

I agree with @SQUEEKY2. I also want to add some food.

^^ Hey,hey^^ I did say a few Granola bars.

@SQUEEKY2 Oh! I thought you were talking about something else :P

Anyway, why not bring something like canned food? You are going for one day, and you need 3 meals too.

I want to add some beer too. Part of the first aid kit. lol
Also, fishing line, hooks, a sharp knife, thermal blanket, water distilling kit, snakebite kit, flashlight and a flare or 3.
I live in the hills/mountains already.
If you get lost and have to spend the night in the wilderness you can stay warm with your foil blanket, rig a fishing pole, have a knife to clean your fish and if you don’t catch any dinner you can drink your 4 beers with a few pine nuts and go to sleep. haha

ganja

Depends on the excursion. Am I going to Disney World or hiking in the mountains? Assuming I am out in nature I would bring:

1. Water.

2. Something to eat. If I am going to be gone a full day, more than 8 hours, it will be more than just a granola bar probably. Piece of fruit, maybe a sandwich, nuts or chips. I’d put the food in an insulated bag to help keep it at temperature for at least a couple hours.

3. Compass if It I am out hiking on a confusing path.

4. My phone if it will work where I am going.

5. Cash, a credit card, and ID.

6. Suntan lotion and sun protection for my lips.

7. Bug repellent.

8. Wet napkins like the ones they give you in restaurants that serve ribs.

9. A couple bandaids and antiseptic of some sort.

Water, fire starting materials, dried foods, compass, solar blanket, signaling equipment of some sort, duct tape, and a few other items, depending on where I’m going.

It utterly depends on where I’m going, what I’m doing, whether I’m going alone, and whether it’s an overnight excursion. The only things I can guarantee I’d take are food and water. I wouldn’t bring half the things other people have listed, especially if walking long distances. I almost always bring a knife, but almost never bother with a first aid kit.

A ticket back to civilization.

For an excursion to the countryside (in priority order):

- Water. I get very thirsty on long walks.
– Food. I get very hungry on long walks
– Clothing for a change in the weather (is that a very British thing to put on my list?)
– Sticking plasters – a blister would ruin my day.

But it could be a day trip to a city or a park/theme park. The list would be different:
– Money
– Clothing for a change in the weather
– Food and water because I’m stingy and don’t want to pay inflated prices

All this in a bigger rucksack than necessary because, beyond a doubt, I will end up carrying my children’s stuff as well

My anti-headache pills

For just one day, not camping or overnight stay? Not half the things you all take.

Food and water, including some high-energy trail mix
A map of where I’m going
My phone and a portable charger (because I’ll be taking photos with it and it won’t last all day on only one charge)
Catheters (because I can’t pee without them)
Hand sanitiser
Waterproof poncho
Cash, bus ticket back to civilisation and my house keys.

Since your question was “a must have” (singular) and not “must haves” (plural), A bag of Keebler chocolate chip cookies. I would also have on my hip my Ruger .22 revolver, but that is not the backpack item that you asked about.

My daily meds, an OTC pain reliever, my family’s medical records (don’t ask) my cell phone, keys, my stainless steel water bottle, my wallet including I.D and credit card and health insurance card, my Non Smart cell phone, some hair ties and hair brush, hand sanitizer, my camera, some type of snacks (granola bars or nuts) umbrella (for rain or harsh sun) something to read (a book or magazine, just in case I get stuck somewhere for a long while) lip balm. Plastic grocery bags or ziplock bags, to collect stuff.

Secondary: allergy tablets, spearmint sugar free gum, baby wipes.

A waterproof
A packet of paper hankies
Lypsyl
Elastoplasts.
They are in my rucksack now and I am good to go.