Agriculture would be difficult, because cereal crops as we know them today simply weren't a thing back then, and it's thanks to human cultivation that they so rapidly evolved into what we recognize today as staples of the modern diet. As for medicine, apart from trying to discourage practices that we now know are actually detrimental, at most you could probably try to grow penicillin, since advance production would be beyond your means. Maybe teaching about harvesting opium as a painkiller, but again that has been know about for millennia, since at least the early days of ancient Egypt, so it may already be common knowledge when you arrive. Really, I think introducing a complex form of writing would have the largest impact, as it'd allow people to share knowledge more efficiently.