If mobility and lightness is your ultimate requirement, the ARLS-Convertu system is the answer.  The Convertu is designed to be both a full-size bagpack, and when you are not carrying much, a waistpouch.  Constructed from tough denier nylon for lightweight, this multi-function bag possess outstanding qualities without sacrificing comfort and usability.  A large 35L volume, front zipped pockets, padded back, waist strap system and front reflectors, the Convertu keeps you mobile at all times. Perfect for outdoor and travel!