The Bravo Bike Backpack can do it all. Carry all of the essentials and more: U-lock, cable lock, spare tubes, tools, pump, patch kit, rain gear, etc. Put your laptop or tablet in the padded sleeve (fits a 15-inch Macbook Pro).
Drop by the grocery store, let out the cross lashing straps -- the capacity of the bag doubles -- and then fill it some more.
Stuffed fat and filled to the brim, this backpack is comfortable. A frame sheet protects your back from pokey things you've jammed inside.
The backpack is waterproof, so everything you've put inside arrives dry.