Tools and flat repair kit in very small stuff sack
Tube
Gloves (thick ones if I'm wearing light ones and vice versa)
U lock
Light cable lock (for front wheel)
An energy gel and a Cliff bar
Laptop
Rain pants and booties, if not wearing them
Rain jacket, if not wearing it
Lunch
Phone, wallet, keys

The bag needs to be reflective as heck, and easy to carry off the bike (shoulder strap?)