An old business messenger bag, roughly equivalent to a California Pak Negotiator briefcase, with a memory foam seatbelt pad for the shoulder strap. Normal loadout is about 3 lbs, but with the extra stuff I was carrying (vest, water bottle, and coffee mug), it weighed closer to 6 lbs.

I'm ambivalent about backpack vs. messenger bag. A messenger is easier to wear over a coat and access the contents while wearing it, and is slightly more professional in a meeting. But, it's limited ability to carry more then a few lbs comfortably hamper it for anything other then office use.