The classic first-hike mistake is carrying too much of the wrong thing. A day pack should be light enough that you forget it is there, and contain the handful of items that turn a problem into an inconvenience.
The things that actually matter
Water, and more than you think. Two litres for a full day, more in heat. Dehydration is the commonest reason a hike goes badly.
Food you will actually eat. Something salty as well as something sweet. Energy bars are fine; a sandwich is better.
A waterproof layer. Weather changes faster with altitude, and being wet and cold is where day hikes turn serious.
A warm layer, even in summer. A fleece weighs nothing and matters enormously if you stop moving.
Navigation you can use without signal. A downloaded offline map, and ideally a paper one. Phone batteries fail in cold.
A headtorch. Small, light, and the difference between a late finish and a night out.
A basic first aid kit, with blister plasters at the top. Blisters are the injury you will actually get.
Sun protection. Hat, cream, sunglasses.
Footwear, which matters more than the pack
Boots or trail shoes you have already worn. A first hike in new boots is how people learn about blisters.
Socks matter as much as the footwear. Wool or synthetic; cotton holds moisture and causes blisters.
What to leave behind
Spare clothes you will not wear. A large knife. Cooking equipment. Anything cotton.
And the pack itself — a 20 to 30 litre day pack is plenty, and carrying a 60 litre one half empty is uncomfortable.
The non-equipment part
Tell someone your route and when you expect to be back. The single most useful safety measure there is, and it costs nothing.
Check the forecast the morning you go, not the night before.
Start earlier than feels necessary. Almost every hiking problem is caused by running out of daylight.
Choose a route well within what you can do. Distance on a map is not distance on a hill; a thousand metres of ascent turns a short walk into a long day.
The honest summary
Water, waterproof, warm layer, food, torch, map, decent socks. Everything else is optional on a day hike, and a light pack makes the day better.
