The Osprey Kestrel is a great medium sized pack for trekking and hiking, or even just general travel and use.  A wide opening with full length zip allows for easy access and packing, and the airspace ventilated back will ensure a comfortable and well ventilated fit.  Features an intergrated rain cover to keep all your gear safe and dry in less than ideal weather conditions, and fit out to take an Osprey hydration bladder that will ensure you stay hydrated.