Keychain style flashlight or headlamp, LED type, not incandescent: The keychain style is tiny and lightweight, the headlamp style is a bit bulkier but easier to use. You can get these for a couple of bucks or less at the local big box hardware store. The LED ones are lighter, more durable and last considerably longer. Very useful if you need to make a repair after dark! I keep a keychain style one as a backup as I normally have a headlamp in my backpack.
Mountain bikers are in different world of repair entirely, one that borders on the comedic absurd. It includes large pumps designed to fill up big, fat tires that squish over things, and a medley of assorted slimes meant to be injected into tubes or tires. In that world, the number of days it takes you to fix your tire and return from the wilderness is a badge of courage—bonus points if you’re bleeding—and we’re guessing that’s not what you’re going for next time you set out for groceries.
Friction gear systems are a lot simpler to adjust, because the movement of the derailer cable can be modulated as you ride, every time you change the gear- they require less cycle maintenance. Modern index shifting systems are a little more complicated because each ‘click’ must release the exact right amount of cable to allow the derailer to move the correct distance to move the chain from one gear to another. Setting up and adjusting a derailer is not difficult and requires only basic tools, but there is a methodical procedure to follow.