Things to consider taking. Some will be a bit OTT for the OP's requirements but they are what cropped up to my mind
Hmmm....
Phone
Things for punctures (tubes, levers, pump, patches etc)
Park tyre boot
Emergency cash and card
General tools. Multitool with hex keys and a chain tool. Know how to use it.
Spare chain link. The right size for your chain.
Miscellaneous emergency fixers. Some duck tape and cable ties.
Charge bank and appropriate cables for recharging phone, GPS, lights etc.
Some food, snack bars, sandwiches, slices of cake, bananas, peanuts etc.
Clothes as weather dictates - waterproof layer, buff, spare gloves etc
Lock
Lights (on bike)
Spare lights. I carry a small front and rear light for emergencies if I'm likely to be out in the dark.
Spare batteries if you have anything battery powered with you (lights, GPS etc)
Mini Leatherman style pliers - often come in useful.
Head Torch