Water bottle,
An additional layer (be it shell, windshirt, smock, jumper, down vest)
Compass
Poncho
Gloves
Midgie head net
Little bag of gubbinz (cord to rig the poncho if need be, spare lighter, tinder, talc)
mini FAK (plasters, alcohol wipes, paracetamol, chlorine tablets, condom, couple of other little things like those micro-toothbrushes)
Notebook and pens
and of course my camera and what I have in my pockets anyway (knife and torch)
Then there are things that are more situational. Sometimes I'll carry a small x10 pair of binos, or goretex over trousers, perhaps a woolie hat and liner gloves in the winter along with an alcohol stove and a 200ml squeeze bottle of fuel.