I use a single rear pannier and usually have
- spare pencils
- small (abut 5 inch diameter) measuring wheel (I usually don't take my 50 m tape)
- hammer
- nails/washers
- paint (I carry spray chalk in the water bottle holder)
- bike tools
- water bottle
The other thing I have that is handy is a small clipboard (from the dollar store) zap strapped to my handlebar with a tube taped to the side as a pencil holder. This holds my waterproof note pad.