While packing for day hike I usually start off with something like the military surplus patrol pack, pack my gear, then decide if a smaller or larger pack will do. One of the drawbacks to this, the patrol pack has two canteens mounted on it. So whether I need two canteens full of water, they...
Depending on how much of a hike or day in the woods this is what I always carry.
Milbank bag (filters dirty colored water)
Miniworks repair kit
Sweetwater Chlorine drops (not in photo)
Us Army Canteen and canteen cup
Open Country Foldable pot