If a smaller bag is sufficient, I really like my Osprey Syncro hydration pack. You can take out the hydration bladder if you don't need it (and in Europe they sell versions without it altogether), and it has an aluminum frame that suspends the backpack off your back for very good ventilation. It's extremely light, and the version I have has a 15L volume. I can actually stuff my laptop computer in it. It's not cheap though.