What volume for what use?
The first element to take into account is the volume of the bag. From 15 to more than 60 liters, the choice is vast.
For a day trip, in the plains or low mountains , a 15 to 20 liter backpack will be more than enough to carry a rain jacket, a water bottle and a snack. If you are transporting the belongings of several people, a 20 to 30 liter model will be more suitable.
These bags most often have a compartment
main, an exterior zipped pocket for valuables (wallet, keys, smartphone, etc.) and one or two mesh pockets on the sides . From 20 liters, certain models offer already advanced carrying systems, in particular with a worked back panel, ergonomic shoulder straps, a chest strap and a central belt, or even hip stabilizers.