I've wasted my time often thinking about the travel to the past thing (and the apocalypsis thing helene mentioned as well), and I felt more or les as jjbrr, I knwo many things, but none of them would be useful just by myself.
I though about using this kind of history in a role-playing game to see what others would come to.
I know a lot of mathematics, wich I could teach to smart people who would listen and could spread the word, and some mechanics and physics, that would be enough to change history, but I doubt I could make many machines to work in my lifetime.
If you travelled farther to the past things in middle age things could get ugly. First of all you are not a Noble, so people won't listen to you whatever you tell them. Even knowing big disasters (like naval battles for Spain), I could hardly get anyone high enough to believe me. And I if not taking enough care someone would send me to Inquisition or some such
. And then , there are no schools to teach people, so things are gonna get tough just to survive.