Algebra and concepts of calculus, and why they’re useful
How to preserve foods
Basic concepts of electricity’s importance and how to make it, but someone would need to explain how to go from raw material to a functional wire, find some rare earth magnets, and figure out how to make LEDs or something else worth using the electricity for.
The scientific method
Concepts of how to engineer/design a solution to a problem
Troubleshooting techniques
Some basic concepts of boat stability and construction
Some concepts of modern psychology
Concepts of critical thinking and rejection of groupthink
Basic physics. Loose explanations of kinematic equations, gravity, friction, pendulums, air resistance, aerodynamics, basic concepts of rocketry and flight/parachutes/gliders
Evaporative cooling? I could describe the concepts of modern air conditioning, but that doesn’t seem useful yet.
I could probably work out how a windmill works, how to make a wagon, how to purify water, how to make water-tight storage.
Germ Theory
The Paradox of Tolerance
How pasteurization works
Fermentation, concepts of distillation
Basic oral hygiene? Habits of at least rinsing sugar out of your mouth afterwards, if brushes aren’t available.
Use of alcohol and heat as antiseptics. Suggestion to use honey in a pinch
Basic concepts of how magnifying lenses work and why they’re important