spare tube, patches, pump, tire boot, tire lever if you need it, plugs if you're tubeless. a little bottle of spare tire sealant is not a bad idea either.
basic multi-tool with bits for most of the bolts on your bike.
it depends on the location and distance you are riding. if you're loaded down and touring into the wilderness many, many miles from civilization, I would think you'd want to bring a lot more. otherwise, the above should be more than sufficient.
preventative maintenance goes a long way. inspect your tires, chain, cables, brake pads, etc. regularly before riding.
the tools I use get used on other people's bikes more often than my own bike.