There are many day packs to choose from. We like a small/medium pack with a bladder insert that will hold our 3.0 Liter Platypus wide mouth. This enables us to fill with ice and water for those 5 mile plus hikes.
Recently we purchased a new men’s Gregory Zulu 30 and a women’s Gregory Jade 28 pack and we love them. They are streamlined, update lightweight pack. They both have cross flo ventilated suspension, improved harness design, and a new lumbar platform that provides better comfort. They have plenty of compartment space also.
Having a day pack will also allow you to carry those first aid items, ankle, knee braces and whatever you may need for that particular hike. Don’t forget extra toilet paper!