Get some basic tools, allen keys, small open end wrenches and various screw drivers. Cutters, Pliers, needle nose, dental pick to remove the orings from parts. vise grips, adjustable wrech. Duck tape, electrical tape, plumbers tape. Silicon lube.
Parts. A set of various orings, etra mask strap, fin strap, Aqua seal. (Basic Save a dive kit) You can throw in a few hoses ( over time you get a collection ) I now have on Big tool chest that has an extra reg, extra octo,, hoses, fins gloves booties, masks hoods, weight and webbing. I also have Bungie cord in a few different sizes. And your friend always..ZIP TIES!!! some gate clips, spare d-rings, 2 inch webbing, spare weight belt, extra weights...
Extra crap: A few lengths of marine grade rope. I use this as a cloths line to hang gear to dry. A few different size tarps. Some for the ground and some for making a sunshades....
you will collect many things over the years...