I have an Osprey 32 backpack and I pack my office clothes, shoes, my lunch, wallet, keys, cell phone, pocket knife, macbook pro, and a 20 oz steel thermos with coffee and a bicycle lock. I also have an azz-saver rear fender, and a small bicycle pump in there, as well as extra sunglasses as I leave when it is barely light and end up riding to the east facing the sun. The backpack weighs about 12 lbs or so, loaded for the ride.
I re-wear the morning ride clothes for the ride home. I hang them off of the backpack in my office space and they are dry when I am ready to go home.