In reply to sean1:
I went for the Patagonia Linked day sack. Reasonably small at 16L but big enough for a rack, water and lunch. Has a strap on the top for a rope. I've resorted to having to clip my shoes to the outside with a carabiner, and helmet clipped on. It does the job and is small, comfortable, and doesn't restrict movement when climbing. So far it's lasted well, but hasn't been abused too much.
Has a water bladder pocket, but not used it. Also has strong attachment points at the top for clipping at a belay. It's pretty comfy so I haven't bothered taking it off to see how useful this is.