I have 2 Camelbak Hydration backpacks - one with a water bladder and one without. This backpak is quite large - you have enough room for a change of clothes, bike tools, food, etc in this. It is padded on the back to give lumbar support to increase comfort. It has a number of pockets. the low one contains a rain cover. There is a zippered compartment on the front for valuables. The next compartment has an open top for quick access. The next compartment is zippered and has, across its back, 2 smaller compartments with a mesh wall. The third compartment is quite large, and has another drawstring wall across it's back. The compartment for the water bladder is behind where the straps connect. It has adjustable chest and shoulder straps. You can store your helmet in the overflow pocket and shoes in the side mesh pockets.
Camelbak Hydrations packs are considered to be the best.
I have 2 of these packs. One has a water bladder and is $50. The other one has no bladder and is $35. I will sell both for $75.