Once you assemble a crude battery, you can in theory begin isolating aluminum (from bauxite ore). I read once that the ancients considered pure aluminum even more valuable than gold, due to its rarity. Electroplating would be another neat trick.
Got zinc and rubber trees? Then you could in theory demonstrate vulcanization of rubber.
You could spur a medical/demographic revolution by (with your first crude microscope) demonstrating, a la Leewenhoek, the ubiquity of microorganisms. Then, if only you could persuade (or get decreed) the “doctors” and midwives of your time to wash their hands and boil/heat their metal instruments prior to surgeries, dentistry, and childbirth deliveries, you would see a dramatic decrease in the death rate. Establishing municipal water-quality systems (even if only as simple as preserving water reservoir areas from livestock and human fecal contamination) would accomplish as much, or more. Ditto for demonstrations of proper food handling. Pasteurization of milk could be next. If your community is troubled by goiter or rickets, then you could demonstrate the wondrous powers of iodide and Vitamin C if you could find sources of those…
You could also kick-start an agricultural revolution, by teaching the basics of selective breeding, cross-pollination, fruit tree grafting, and maybe even artificial insemination.
If they have crappy roads, or no roads, I could create a sample bit of macadam road. Ditto for wheels/axles, some simple toothed and racheted gears, and the basic wing shape (a “puddlejumper” would be a do-able demonstration device).
Brass instruments, and to a lesser degree, the woodwinds and strings.
If your community was really primitive in its economic arrangements (as in bartering agro commodities), you could unleash much economic potential by introducing a controlled-supply, standardized currency/specie combination, as issued by the king’s treasury (an Ur-central bank), and establishing means for borrowing and lending for non-usurious interest percentages.
If they don’t have it already, show them the Arabic numerals and start teaching math – as much math as you can remember.
Don’t remember the Periodic Table of the Elements? Well, jot down as much of it as you can remember, along with whatever you remember from chemistry, etc. Set down the general principles of the groupings along with some ramifications of their significance, and let others fill in the blanks over time… just the basic concepts would be revolutionary enough.
I bet most of us could help steer the course of basic-patent-yielding research… towards lightbulbs, piston engines, pumps and compressors, batteries and generators, aviation and rocketry, etc. etc.
One more thing: I’d have to take a whack at producing the sweetened chocolate (access to cacao beans necessary), graham crackers, and marshmallows for s’mores. Living by campfire (or the indoors hearth equivalent) with no s’mores – or at least toasted marshmallows – just wouldn’t be as much fun!
