Well, think about what might happen . . . Common things occur commonly.
Hoses work loose. Wrenches that can tighten them are nice. You might forget to put a hose on, or back (for example, coming home from the tropics, and forgetting to put a dry suit hose back on). Allen wrenches are required for that. A ScubaTool is a gadget that combines those things, and they're pretty handy (they also have a screwdriver tool and a knife).
Mouthpieces can come loose, and zipties can cure a myriad of ills. Double ender snaps can help secure errant octos (rubber bands are part of this as well) -- buy brass ones if you are going to share them, because they are much cheaper.
Leaks plague all dive gear. Aquaseal is great stuff in a lot of circumstances. Duct tape is pretty handy, too.
If you start to dive a LOT, or go to remote places, an o-ring pick and a set of replacement o-rings is awfully handy, and uses up very little space.
Spare batteries for anything that is battery-powered and permits the user to change batteries is a good idea for any dive you don't want to scrub.
Spare gloves are awfully useful. You can carry spare dry gloves, if you use them, or default to wet gloves, which you can share with anybody who forgets theirs.
I carry mask straps and fin straps, because I don't want to lose a dive because a buddy has broken one or the other. I use slap straps and spring straps, so I won't need my spares
The longer and the more you dive, the more things you'll put in your kit. There'll be more in the car, a little less on a boat, and probably quite a bit less on a plane, unless you are going cave diving for a week, in which case your set of spares will be equal to about what the normal diver carries in basic gear for a week's diving
