It has a removable waist strap for stability, adjustable backpack straps, and is waterproof! Plus, it can expand to fit 44L of capacity, but can compress to fit 38L.
Promising review: "I just got back from a nine day trip backpacking Europe and could not be happier about this backpack. When first ordering I was worried that the backpack would not have enough room and the overall quality of the bag would not be as good as others, turns out I had nothing to worry about. By rolling my clothes up I could fit everything, including two pairs of jeans, nine T-shirts, one long sleeve shirt, one down jacket, two basketball shorts, one belt, one beanie, six boxers, six pairs of socks, a hefty cable bag, a 12-inch laptop, sandals, and toiletries. There was actually still room for more! I had no issue bringing this as a carry-on and storing it in the upper bins of British Airways and Aer Lingus flights." —ast89
Get it from Amazon for $74.99 (available in six colors).