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.
Chrome Citizen Bike Messenger Bag
Chrome Cardiel Fortnight Bike Backpack
Chrome Victor Small Utility Bag