I have an Osprey FlapJack:
http://www.evanscycles.com/products/osprey/flap-jack-courier-bag-ec022594
It has a laptop section, which is actually useful for folded clothing, keeping it together and upright.
Then there is the main section, which is big and open and small things will get lost in it.
But there is a section in front of that, which is not as big as the main section but has two netted sections suitable for carrying the small stuff.
Then there is another zipped pocket at the front of the bag, with a hooked clasp for keeping your keys safe.
And finally another, slightly bigger, zipped pocket on the left side, which will be to the front and accessible when you have the bag over yours shoulder as normal.