Choose a topic that is difficult or challenging to explain (“to explain” is not necessarily to “defend” as students are required to do in writing an argument paper). For this specific purpose, some good topics include hoaxes, unusual sightings (e.g. UFOs, bigfoot, the Loch Ness monster), the supernatural or paranormal, with one crazy online resource that gives us plenty of opportunity to apply abductive reasoning is the Museum of Hoaxes at http://www.hoaxes.org/ or apply abductive reasoning to events that are the subject of conspiracy theories, unsolved crimes or other court cases, etc.
After you choose your topic, explain the topic you chose and why it is interesting or controversial. Present good arguments on more than one side of the issue (e.g. competing explanations of the facts). Analyze both arguments that you have presented. Then present your own argument for your theory that you feel will best explain this phenomenon. Are there any holes in your theory? Is there any information that would be likely to strengthen or weaken your case?