But the question arose, what would the best modern idea that three modern people could bring to upgrade agriculture and so living in a D&D (sorta) medieval universe? They have crop rotation -- maybe contour plowing? They are at least aware of the circular problem that the people need more food to be able to build the infrastructure to increase agricultural productivity.
And then, what is the best thing they can bring in as their own D&D characters? The decanter of endless water -- endless potable water -- would probably bring about the most change if they could only get enough. But both objects and spellcraft are aimed at adventures.