You didn't define if you were referring to daily items carried when commuting, or items carried for weekend rides, or long distance travelling.
If just daily commute, just the essentials - wallet (cash+license+road tax), handphone, house keys. Unless planning on picking something up, then backpack tags along for the ride.
It's going to be more or less the same for other type of rides, just that for longer distance ones we would usually have a larger bag/panniers/etc which then allows us to carry more emergency stuff, like a tire puncture repair kit, lube, cable ties, tape, tool kit, etc.
If you have good space under your seat, always good to have a tire puncture repair kit, cable ties, a small tool-kit (bikes usually have them), and some electrical tape, if you have more space you can get creative to prepare against more potential fault/failures a bike might encounter while on the road.