Whilst a backpack is a necessity for longer expeditions, a good quality waist pack or day bag can be just as important for shorter treks.
Prepare for the Unexpected
On any day trip you should always pack with the consideration that conditions may change. Extra layers, waterproofs, hats and gloves will all be a godsend if the weather turns in Winter. During the Summer water, sunscreen and sunglasses are just as important.
Additional space to carry a snack for when the walk is a bit longer than you thought is a good idea and a camera to capture your memories is also very useful.