Any topic that you are comfortable to speak about in front of others for a good few minutes should suffice. Nobody really cares what the topic is, so long as you present it in an erudite fashion and are able to answer questions about the topic in question with confidence. I did one about my disability (and it was composed of half-ass strands of research), but my class seemed to bear it, and after answering a few questions, it was over. I somehow mustered a distinction. You'll find that it is actually very difficult to perform poorly in the speaking endorsement.