Fit lights to your bicycle. I have a red flashing rear light underneath the saddle and switch that on even if I ride in the middle of the day – even more so on an overcast day. If you intend riding at dawn or dusk, add a steady, white front light as well.
For the rest of the kit, use your common sense: If it looks like it could rain or get cold during the day, take along something waterproof and/or warm: You cannot adequately control your bicycle if you are shivering with cold. Sunglasses (or clear glasses in the evening) will protect your eyes from dust and the like blowing into it – again, it is difficult to control a bicycle if you are riding down a steep hill, traffic next to you, and something blows into your eyes. Wear the right shoes: Slip-slops or sandals may be acceptable for a very short trip to the café on the corner, but have you thought of what would happen should the sandal get caught, or you get involved in an accident? If possible, rather wear closed shoes (again, make sure the shoe laces don’t get caught in the chain or chain ring) or – even better – MTB cycling shoes that will allow you to comfortably walk around when off the bicycle.