My current "save a dive kit" is actually broken into 4 different small packages.
In my reg bag, I have a spare long hose, spare high pressure hose and spare inflator hose. I also carry a spare 1st stage and spare 2nd stage.
In my toolkit which is a Pelican 1050, I carry two adjustable wrenches, a metric and an imperial allen wrench sets and screwdriver with room to spare. I also have xeroxes of my DAN insurance card and a copy of my cert card in this box.
In the parts box which is another Pelican 1050 drybox, I carry spare hair bungies, Voton o-rings, o-ring pick, o2 grease, zip ties, a few feet of bungee, a few feet of 25# line, some spare 1st stage port plugs, 2 spare bolt snaps, 2 spare d-rings 2 spare webbing keepers, spare C cells, Dramamine and Ibuprofen. I also store my zipper wax and KY in that drybox as well, making it a tight fit.
I am going to add a cannister light service kit with o-rings, various parts and spare bulbs and a drysuit patch kit.
Lastly, I have a small pouch that is my "general" kit which I also carry on hiking/rock climbing trips that has an electric glow stick, first aid kit, flint/steel, magnesium block, leatherman, leatherman accessory bits/bit holder, small P3 Surefire light with spare bulb/batteries, various different surefire filter heads, spare AA batteries, and rubber bulb to squirt air.