Showing up prior to the Reconquista in Europe with a working knowledge of 0 and the Hindu–Arabic numeral system would go a long way. You could probably also figure out distillation. If you aren't too early you could probably invent telescopes, microscopes, and germ theory. And that's without bringing anything other than knowing that those things could exist.
There's probably a good book on practical engineering stuff that would really come in handy. Simple recipes for things like clear glass, gunpowder, interesting properties of coal, navigation, etc.