Others have covered the tool/skill question, but I thought I'd chime in with the points that resonated because they are also pretty straightforward to learn how to do:
-chain tool (can fix damaged/broken chain, obviously; can also use to render bike functional if DR breaks/malfunctions-- chain will need to be shortened)
-spare spoke(s) and spoke tool
-spare cables- brake and DR
--lube (squeeky chain is annoying and hard on your drive train)
--boot material/spare tire (if tire rips or tears)

Also super handy to have on hand is duct tape (wind some around a pen to make it easier to carry)-- it's amazing what you can do with duct tape and it saved my ride when my glasses snapped at the nose piece as I crested a mountain pass in the rain on a recent tour (I am blind as a bat and had forgotten my spare glasses).

-a couple of zip ties-- plastic doohickies, can use to lash things down and as emergency fasteners.