In my messenger bag I carry: A tool kit (various hex wrenches, tire levers, a tube, a patch kit, a knife, a pump, some zip ties, some tape), an energy bar of some sort, pants, shoes, a shirt, underwear, usually a hat (probably a cycling hat), two rear lights and a front light, a cable lock, keys, wallet, a book, my phone, rain jacket, some sort of plastic gallon bag incase it rains for extra water proofing, and an extra pair of socks. I carry my main lock around my waist.
And its a Girl Bike Dog brand messenger bag... and I lost the sternum strap forever ago so I use one of the straps in the back to tie it around front.