How about a workout design workshop? You can discuss how much cardio, strength training, flexibility work to include in a daily/weekly program for the average “older” client. How they can progress their workout, focusing on all body areas.
Or if you have an event in your area coming up, how to train for that event? A 5K walk lets say.
You can also poll your audience to see what they’re interested in learning more about. You may get some better ideas from them that are then geared right toward the audience you’re trying to reach.