I use a messenger bag, I will say get a good one. The cheap ones put the weight on your shoulder when you ride.

Backpacks can be ok, but I have not found one that fit just right, but if you buy one made for cycleing you will get one that fits right.

Sounds like you need a handle bar bag to me. Maybe a seat bag as well. The two should be able to hold your lunch, and tools for flats and such, a lock and probably even a change of cloths if the handle bar bag is big enough.