Honestly, it doesn't take very much. Cables and cable housing are good things to have on hand. Frequently what looks like is going to be a simple fix turns into a damaged cable issue. I use slick galvanized cables so that I can solder the cut ends.

Nuts and bolts aren't something that I'd worry about except for not throwing any away. Uh - always work on your bikes someplace that the stuff that you drop doesn't have a place to hide. If you have bikes with linear pull brakes, the noodles that join the cable to the brake arms are good to have on hand.

How are you holding up your bikes while you work on them? I've got some recumbents that don't fit on traditional bicycle work stands. A fixture for conveniently holding them while I adjust the derailleurs is worth it's weight in gold.