I write guides and make YouTube videos about and for hikers and would love to hear from the group what items they never leave home without when heading out on a day hike. Here's my last backpack video
First aid kit, headlamp, extra layer, water, food, whistle, and ID. I also think it's a good idea to hike with a knife and a lighter or matches, but I don't always follow my own advice on that one.
Water filtration (Sawyer mini), basic first aid, something to write on/write with, emergency blanket, knife, cordage, and fire-starting kit are the things that never leave my daypack. There's probably some other stuff floating around in there, too, but it varies.
Oh, one other "must have" that is nearly always in my pack: a small 10x power monocular with a cord to wear around my neck...
Always the ten essentials. See the REI site for a suggested list.
1. The 10 essestials should include matches, a whistle or mirror, & toilet paper in a plastic bag ( which can also be use to start a fire).
2. On a hot day the water should be doubled.
3. Learn how to use a compass before you go out.
4. No need for the saw. You should only used down, dead wood. If you can't break it on a rock, it's too big.
Know how to use a compass AND know how to read a map, especially a topographical map.
Items not in pack but on my list to bring on every hike
That's my list from our checklist in our gear room