I know what you mean. I remember going around in circles as a trainee teacher.
As you are a trainee teacher, you should go easy on yourself. I would suggest not inventing any sort of presentation activities. I suggest you get hold of a good course book, whichever one your school recommends, and just base your lesson on that. You can also use Raymond Murphy's books (English Grammar in Use) at the appropriate level. These will give you inspiration and ideas for how to present the passive voice.
If you want to write in again with your suggestions for presentation, I'll be very happy to comment on what you come up with.
All the best,