I have gone back and forth on this.
Today's rig:

Front and back lights (the tiny strong kind)
Reflectors on the spokes
Bottle and cage
Bike lock (Just a cable kind, lighter but not for overnight or dark alleys)
Small under seat bag for ID, $$, First aid kit and room for my cell phone if it rains.

No tools, no tire repair stuff. (I am terrible at tire changing, so I just call a buddy)
Took off racks, fenders and kickstand.