Explaining how to do something is, perhaps, the most obvious kind of demonstration speech. There are countless everyday skills that are useful to know, such as how to remove stains from clothing or how to get whiter whites when you do your laundry. If you can come up with something you can demonstrate, that your audience doesn’t know much or anything about, you’ve got a good topic for your speech.
You can show audience members how to follow a recipe to make excellent omelets or cupcakes, or whatever your culinary specialty is. There are many computer projects that could be demonstrated. Many crafts and hobbies that involve making things also can be demonstrated in a short speech.
Just about everyone wants to know about how to be more inter-personally effective. How to run a meeting or discussion is useful information that a demonstration speech could provide. To improve their social lives, some people would like to know how to start a book group, or an open mic at their local café.