Light and compact, hip packs (or fanny packs as they are often referred to) are the perfect solution for carrying all the essentials of a trekking trip. They are especially useful on short and warm-weather treks as you do not have to bother with a backpack. The Fjällräven hip packs are also designed with clever storage details including key clips, straps to attach packs to a jacket and inside pockets for secure money and ID storage.
As to what you store in your hip pack during a trek is entirely up to you, but seasoned outdoor enthusiasts are likely to suggest you consider using it for holding onto high-value items. You can also pack these bags with things you need with more regularity during a hike. Like snacks, a collapsible cup, and a map.