What should you put in a day pack?

It depends on how you're going hiking.

If you're in a team competitive hike, you'll each want a coat, flask of water/coffee/tea (whatever floats your boat), a ration pack for emergency (high sugar foods + nuts), gloves, hat and a scarf, a torch, a whistle. Then food for meals.

If you're just going for a leisurely stroll; food and drink with a mobile phone in your pocket should do it.