I was given one of these REI Ultralight day packs for a gift once and now I use it for almost all my day hiking in the mountains, bike rides when I need to carry some gear, etc.. It weighs next to nothing but yet is strong, has good shoulder straps and it even has a water bladder compartment. It is also cheap. I have a whole stack of backpacks on my basement shelf but this is the one I use.
http://www.rei.com/product/747522