You're likely to be asked why you're interested in the course, just to make sure you're not freeloading to get into Cambridge! If you've done any reading before the interview you'll probably talk a little about that (I had a discussion about the Mabinogion with my interviewer), and also about the papers you might take for Part 1. Apart from that they'll be throwing hard questions at you just to see how you think; to give you some idea, I was asked how modern scientific techniques can help with Dark Age historical research and how you could ascertain the history of a manuscript from its physical condition.
I think everyone comes out of their interviews feeling they've done badly. They don't expect you to know the answers to these questions, they just want to see how you try to answer them!
I sent two history essays and a French essay, but I was doing History, French and Spanish A-Levels so my hands were tied. If I'd done English or another essay-based subject I'd have sent an essay from that instead of French.