What I take varies between home and travel.
For home, DIN O-rings, a ScubaTool, zipties, a set of Allen wrenches, a Pro valve insert, some cave line and some silicone lube seem to suffice.
Travel, I'm kind of with AJ, although I don't bring a whole spare regulator. I do bring a whole spare canister light if we are cave diving. Matches, a voltmeter, spare Anderson connectors, spare boltsnaps, spare inner tube, a spindle . . . the SAD kit has grown as the list of things which have delayed or prevented dives has lengthened.