My basic kit is: fenders, CREE front light (after almost hitting an idiot walking at night in dark non-reflective clothing), dual blinking tail lights (want to make sure motor vehicles see me), rack, panniers, water bottle cage (I just use 20oz bottles, San Jose water is not really drinkable), mirror (the old neck is not so flexible).

For longer rides, beyond the range of close public transit, add pump, tire levers, spare tube, patch kit.