I'll preface this by saying I'm a Software Engineer who works at a desk. However, we do have a Commissioning Team that goes to client sites to install the hardware/software that we develop, as well as other Electrical/Automation Engineering duties.
As far as topics go, we used to not only talk about safety topics that could affect us directly (ergonomics, washing hands, in/out procedure), but also other members of our team (pinch points, 3 point contact, client site in/out procedure, client speed limits, stay alert while driving).
Weather conditions (falling snow, bears waking up from hibernation) and accidents reported in the news (driving according to conditions, safety goggles) are also good topics, especially since they're current.
Using these greatly increases the number of topics, to the point where repetition happens at a point where it is considered a "refresher" instead of "the same thing again".
The one thing that surprised me about your post was that the employees were required to come up with and speak on topics.
I've normally seen this task reserved for the Health and Safety Rep or a Manager, as it is their primary duty to ensure that their staff are working effectively and safely. This way the person talking about the topic has a vested interest in it, instead of doing it because they were told to.