Combining the criteria of being ‘entertaining, informative, persuasive, and inspiring’ is likely far easier than you think it is. It would probably help to break your speech down to its basic elements. First find a topic about a person, place or thing that is highly interesting to you. Your passion and enthusiasm on the topic will go a long way in crafting your speech.
Using the basic format of including an opening, body and conclusion, think about what your purpose is in presenting the speech. I always recommend thinking about your conclusion first. What is the takeaway that you want your audience to have? If you come up with a strong conclusion, perhaps a call for action, it gives you direction to go to lead up to your call for action.
The informative part is where you provide some details about the subject. A basic format is to make a point, tell a story, then make a point. The number of times you follow this format depends on how long you have for your speech. Your stories should have a purpose, related to the theme of your speech and not be filler. The part where you make your point is where you are persuasive and your stories should add the entertaining value you want to add.
I believe that many speech writers spend far too much time trying to create extra special speech topics, when we already have many stories within us.
As an example, I have speech that I have given different variations of over the years. It is about a fellow named Bob that I used to work with. He was a handyman, i.e. a do it yourselfer, but everything that he did or touched, went wrong. I have used his story for themes of time management, determination, things that go wrong, strategic planning etc. All the same stories, but the points in between the stories are crafted to persuade and inform reflect on the message I want to get across.
Good luck with this and future speeches. Thanks for your question!