A full sized backpack is not necessary for day hiking.
A small, lightweight daypack with a waist strap is sufficient.
Do not purchase a daypack without a waist strap that secures in front of your waist with a clasp. By adjusting the front shoulder straps near your shoulders, a waist strap allows you to adjust the weight of the daypack from your shoulders to your waist as your need for comfort dictates. Or you may wish to adjust the weight equally between your waist and shoulders.