I second the pannier suggestion. I rode with a backpack for years, so happy to get that stuff off my back now (actually, I still carry a backpack, I just put it in the pannier rather than on my back now).

If you're sold on the backpack idea though, I'd consider just getting a regular backpack + water cage & water bottle, or regular backpack + Camelback bladder stuffed in the backpack (replacement bladders are available at any bike shop, just stick one in your backpack and away you go). I'm not aware of any Camelback big enough to carry laptops, books, etc.