I would put at the tops of the list – a spare quick release, followed by a stone bag and then a case.
The spare quick release almost goes without saying; but i have to say it since many pros often do not
have a spare quick release plate stashed in their gear bag. And a tripod is almost useless without a quick release. And that goes without saying that if your tripod does not have a quick release you can add one to it ( click here to see one version)
The stone bag is next because it gives you a place to stow small items, or a shelf to use for holding things. Changing lenses, filters, holding the light meter, your cellphone etc, etc. It can be left on most of the time and will fold up with the tripod.
Manfrotto makes a utility apron that is larger than the stone bag, and has a couple of dividers in it, so you gear doesn’t have to rub or mix. Keep your lens and your lunch separate.
The tripod case is a person thing. While I don’t use them in the field, I carry my tripod to the site in a case. I like to store my tripod in one since it has my spare quick release, stone bag and a little tool kit. The tool kit has a screw driver and a socket that fits my tripod and head, a roll of 3M blue painters tape (sticky, easily removable and leaves almost no residue) and some fluorescent pink nylon string. The string is easily seen and is some times needed of all sorts of things. A buddy of mine, Ken, also stores extra memory cards in his tripod bag, so if he ever forgets or runs out of memory, he still has a spare. Great thought.
Lesser know is the cable clip that used to be standard on every Bogen and Manfrotto tripod. These clip to the leg and you can use them to keep your cable release or other cables off the ground and away from your feet.
When you are using you heavy lens a lot with your monopod, the monopod pouch is a great accessory to have in you bag. Slip it on you belt and it allows you to easily walk around with your gear, podded up. No worrying about the stick getting in the way, you let you hips carry the weight on you belt. This is heavy duty corder nylon. I don’t know if you can wear one out. It is that heavily constructed. Click Here for more.