I'd add one other to Mike_H's list, which was not mentioned and I just thought of, but might very have you changing the course of history: advise someone on how to make war, if you have any concept about creating weapons or, possibly, tactics.

If you are the guy who knows how to make gunpowder or any explosive, then you risk being burned as a witch or something. But the upside may be that the King hires you to create a whole bunch for him.

I think you'd need to get past the first few months to be able to use this.