What are you most interested in?
What are you curious about? What do you wish you knew more about? The life cycle of Monarch butterflies, the origin/evolution of modern holidays, and how to make your own fresh, ground coffee could all make interesting discussions.
What are your passions?
Often people are extremely engaging when they discuss things they are passionate about. How to live/eat healthier, how to train for a marathon, and ways to improve your writing are examples of passions/hobbies people have that you could turn into a speech.
What is an unusual or interesting process you could describe?
How a Rolls Royce is made from start to finish, the steps involved in building the Eiffel Tower, and how the U.S. Constitution came about are all processes that would be detailed and interesting to discuss.
Find something you love, want to know about, or that you find interesting. Describe/discuss it in a fresh, new way. Entertain your audience, and enjoy yourself!