Lightweight modern backpacks are ideal for a day out in the mountains or forest walks. A 30-40ltr day pack will easily take enough gear for a days hike, be sure to put your waterproofs and spare clothing into a dry bag to ensure they stay dry for when needed or to put wet clothing into so you don't get other gear wet inside your pack.
You may be looking for something a little more heavy duty that offers some camouflage. The Kombat UK rucksacksare designed for military use and a very popular amongst the bushcraft community and are much more hard waring. They come in a range of camouflage colours and patterns and are loaded with extras featuring many compartments and pouches for easy storage and quick access.