You probably won’t agree but here is how I see it...
I honestly think you are going about this back to front. You should be thinking about what you have to achieve and techniques before worrying about content. You need to be able to GRAB peoples’ attention straight up. You also need to give them a clear overview of what you intend to talk about. Once they are interested in what you have to say, you have to maintain that interest for 5 minutes of talking, which is a lot longer than it sounds. You need to round off what you been saying – a quick summary of the important things they should have gotten out of you talk. Every now and then you need to challenge your audience to think – this keeps them involved. You can point to people and rhetorical questions e.g. “What do YOU think? And YOU or YOU?” The question could be anything e.g. “What do you think YOU would do?” or “How would that make YOU feel?” or “Do YOU think that would be the right thing to do?” Would YOU care?” etc.
If you are nervous about looking directly at people’s faces, then maintain your gaze just above the faces in the back row. You appear to be looking at the audience that way even though you are not. Do move your gaze around from side to side as it is rude to stare.
This not a test of your ability to stand up and waffle for five minutes. It is a communication exercise to determine your ability in being able to effectively communicate to a large group. So your talk needs to be well structured and well practiced. Practice giving it at home 5 to10 times.
I would be googling “great public speaking techniques” “best topics for public speaking” etc.
As for topic, I would pick something everyone is familiar with e.g. cats and your talk might be on compulsory sterilisation of all cats; compact fluorescents and the fact they contain mercury and are not being recycled; cats, dogs and budgies and the dangers of children getting diseases from them; etc. If you think you can pull it off with a reptile topic, then go for it. Just remember not to use technical terms and hobby jargon that others will not know the meaning of.
Blue