A spark plug (or two) and an appropriate wrench. Calibrated torque wrench for bonus points with the employer.
Failed plugs aren't much fun (espeically at 100ft after takeoff) but quite resolvable. The cooler-to-touch is the one to replace.
Depending upon the aircraft, some rudder springs are good too. Plus some coins/washers to add to the springs to make them easier to spread.