Honestly if I had my current level of knowledge, probably hygene. Teach them to make soap (animal fat and a source of base like ash), wash their hands, keep poop away from potable water sources, stuff like that.
Remember, it literally took until Victorian times to figure out that washing your hands prevents disease.
Also, math. Teach them how to do basic arithmetic, how to use a unit of measurement to figure out how big something is, stuff we'd learn in elementary school but weren't rigorously developed until the Ancient Greek age.