I always carry a second skin with mine, I cant fit my laptop into my backpacks pocket with the skin on, but I carry it around just incase I need it. I also carry a thin flexible mousepad for my mouse.
A pair of earphones, because headphones are too bulky. A portable HDD. And a few flash drives, its saves battery when watching movies etc off a flash drive, and using a portable HDD for that is just a hassle with the wire and all.
And of course, a secondary battery.
I have a Wenger backpack and love it.