>>185399I really like the CamelBak M.U.L.E. It isn't the absolute lightest pack, but you can carry 3 L of water plus some stuff. It is $100 at REI.
I'd go with that, or with the Osprey Talon 22 (on sale at REI right now for $74.99) plus whatever hydration bladder you like. My top pick for hydration bladder is the MSR DromLite with the added hose and mouthpiece accessories. I've seen Platypus bladders get punctures and burn holes whereas the MSR ones are lighter and seem indestructible.