i also am on my bike all the time. mostly i ride to and from school, every day. it works out to be right about the mile mark each way. plus i ride my bike everywhere else i need to go within about a three mile radius, sometimes farther. when i am doing serious shopping or retrieving of heavy stuff (like lumber or metal) i use the automobile.
so, becuase most of my travel is between home and school, my bag commonly carries papers that i need overnight for homework, (to school) sandwhich and fruit, various small things like a chunky sharpie, sunglasses, a pair of socks and fresh running shorts for track practice after school, etc.
when i am going anywhere besides school, i skip the papers and fresh running shorts. i mostly keep my bag empty so that i can fill it up with whatever i buy or find or whatever.
normally i carry any of a number of shoulder bags, mostly no-name cheapies, or from various clothing stores. right now my favorite is from the gap. its actually like 6 years old. for heavier loads i carry a proper backpack, sometimes something with a little suspension/weight distribution.