I used to carry a lot more than I do now, but at minimum I have:
- A spare set of plugs, with a plug wrench
- multitool style screwdriver - most screws are phillips on my 750, I like to have something to snug up something that might have come loose, like a carb boot, or I need to get at the ignition or behind the clutch cover for whatever reason.
- 10/12/14 wrenches. That covers most of the bolts on my bike. Not all, but most...
- Allen wrench. I changed all my engine bolts to stainless allen bolts, so I bring one incase I need to get a cover off
- Spare fuses
That's about it. If your bike is in pretty good tune, no need to carry an entire tool box. If I'm touring, that's a different story and I bring more stuff. But going out for a couple hours? Easy to overthink it.