I have a low profile camelback that i use occasionally with my gopro. it fits under my coat and no one knows. It carries:
swifter gopole that collapses into three pieces
all my applicable gopro accessories
maybe some gum or mints
I try my very best to keep it bear minimum. No one wants 10+ extra pounds hanging off their back. It throws your mass off more than you think.
If its an extra cold day, ill throw some precautionaries in there(warmers, liners, etc)