Some great ideas so far. Leveraging your current clients is a great idea – talking to them and find ut waht they love about working with you, and any pain points you could improve could help you optomise what you already do. This will make your current clients more sticky and increase the likelihood they will recommend you to their network.
I definitely agree with a kick off group type activity. Post lockdown baking shape up?? It is a great way to kick people into a new routine and add a social element to exercising, which you can then leverage into new one on one clients as well.
You may also want to think about some specialized offers that target specific types of clients and use FB to speak to them in a really targeted way. It would mean some trial and error but might assist in identifying specific groups you could target for the future. For example, post baby re-establishing fitness, rehab class, executive fitness. Or even choose and event and help people train for it. It is really important that your images tell your story in a really appealing way. Maybe go back and look at what you did last time and identify where there might be areas you can improve; images, headlines, offers or targeting? And ask your current clients how they found out about you? you can do this informally through a chat or even design a short survey and ask them for a range of insights which you could then use to target your activities.