Your competent communicator book gives you a clear goal for each speech. Only the first speech, the Ice Breaker, also suggests a subject. Yourself. For the rest of your speeches, you're on your own. Narrowing your topic choice can be hard, so here are the questions I ask my mentees to help them decide.
What are you passionate about?
Bring that emotional focus to your writing. Your own interest in the subject comes through in your presentation. Some people may feel caught in a trap of always talking about the same thing. Others may want to keep that focus. There is room to explore topics both broadly and deeply. Pick one, and you can keep coming back to this well.
Are you making a point with this speech?
The third speech is all about getting to the point. You should be able to sum up what you're talking about in one sentence. Not all speeches necessarily need a thesis statement, but without a summary you may not know where to start the direction yet. You could be telling a story where the point is to relay what happend in an amusing way. One of the things I like most about Toastmasters is learning from my fellow speakers. If you've decided what you're talking about, it could be a broad topic. Narrow that down, decide on what the point is, and then you can start building an outline for the speech.
Do you have a personal goal for this speech?
If you're working the CC (Competent Communicator) series, they each have a goal to work on. Figure out how your content will support this goal. Thinking about body language, you can craft something with two points, and plan on playing both sides out physically. Working on vocal variety? Write in some exclamations, and where you will speak quietly or take a pause for emphasis. Planning these out with your content help you incorporate them into your speech more effectively.
Do you already know enough about the topic to talk about it?
Some people speak very easily off the cuff, where as other people need time to research to feel comfortable. Just keep in mind, don't get caught in the trap of putting off a speech because there's more to learn about your topic. There's always more to learn!
What do you want your audience to take away from the speech?
Not every speech needs to be poignant and end with a maxim, but that's one option. Maybe you just want people to laugh, that's a valid take away too. Decideing this helps you craft the conclusion.
What questions do you ask yourself when you're writing a speech?