What do you keep in your toolkit? What spare parts?

I have carried low beam bulbs since it's dangerous without and they are relatively easy to change. Also a small flashlight.

 

nothing. except spare tyre, jack and 2 x 12 base, wheel chocks, and lug wrench.

 

An old shower curtain to kneel on while changing tires. A 30" bar with quality socket to fit wheel lug nuts. A X bar lug nut spinner. A set of super quality long jumper cables left over from when I would jump a car a month in prior cars. A reflective triangle. A yellow reflective vest. A "can" of motor oil. Some rags. Spare wiper blades (the last set removed when I change them ... probably 4 years old but save your butt if the ones you are using fly off). A couple of old towels and washcloths for use as rags. Some flares. 2 screwdrivers and an adjustable wrench.

At 73, I still help others.

 

Small AA LED flashlight, a full roll of paper towels, an old heavy shirt, all stored under the rear floor.
Used these over the years to change tires, wipe hands, absorb spills, help other motorists, etc.
Better than a dealer-sold first aid kit!!