robertorolfo

Try a tennis backpack. They are reasonably priced, hold a ton of stuff, and usually have a lot of useful sub compartments. They won't be completely waterproof in a downpour, but as others have stated you should be using secondary bags for valuables anyway.

Also, whenever it's raining if you ride fast enough, the rain will never even hit your backpack. Yes, I'm kidding, but it's good motivation.