I carry a MEC Blitz hydro pack... it is designed for rock climbing and is a remarkably well made and tough little day pack.

It's compact at 12 litres, cost a whopping $20.00 and if I can't carry what I need in this, I use my paniers.

I usually carry a shell jacket and pants, snacks / lunch, tools, my camera, my pump, and also carry a 1.5 litre hydration pack (not included) and still have some room to spare.