Let’s take a three-pronged approach: I’ll go back to the 1950s and channel all the money and manpower sunk into the Interstate Highway system into passenger rail.
I would…
…alter the importance in the early days of product development to reduce packaging waste.
…along the same lines, educate (not fear-mongering) people early about the reality of a disposable society.
…focus on high speed passenger rail systems like those found in Asia
…focused on alternative energy resources
It would be a bit of a gamble, but I suspect that the thing which most changed the world (for the better) was the proliferation of paper. If you could go back to the first civilizations and teach them all how to make paper, the advancement of societies and technology would be–literally–millenia beyond what it is now.
If it’s a swoop down type of thing: would have given Native American communities immunity to European diseases. This would have been an immense change in the world as we know it.
I’d go back to Augustan Rome and explain the Scientific Principle and the decimal system. That way we get the Industrial Revolution out of the way and we’d now either be living in much better place or a blasted wasteland.
Here’s some I’ve posted in previous threads:
[ul][]Go back in time about two million years and transplant colonies of early hominids to the Americas, southern Asia, and Australia. This would accomplish two things: the fauna of the areas would then have time to adapt to (initially) inefficient hominid hunters like African fauna did, so the human-driven extinctions either wouldn’t happen or be not as bad. And secondly, it would give genus Homo the chance to remain diversified, and we would have been less apt to think of Sapiens as a unique divine creation (and less apt to denigrate sub-populations of Sapiens).[]Give the Romans living at the colder northern frontiers of their empire the inventions that later made northern Europe economically prosperous: the horse collar and the horseshoe, the moldboard plow, knitting wool, the horizontal loom and the spinning wheel, and the chimney and mantle fireplace. Civilization, and even better a thriving tax base, help prevent the collapse.[]Keep pushing Norse settlement of the New World until a viable colony gets established. The native Americans get a chance to acclimate to European diseases, and borrow European technology (ironworking, cattle, horses, etc.) before the invention of firearms gave Europeans the means to overwhelm the natives. The Aztecs would NOT crumble like powder because a weak superstitious emperor thought the whites were gods. For that matter, if the Aztecs gained cattle and hogs from Norse traders, they might not have had the ritualistic cannibal/sacrifice culture they developed. When Europeans finally did develop true ocean-going ships and guns, the native Americans might have resisted at least as well as the rest of the third world.[]At the dawn of WW1, show the surgeon generals of both sides that a compound first synthesized in the late 19th century, DDT, could kill body lice. If nothing else, the vast number of disease related deaths from that conflict would be reduced. [/ul]