Ball and passing is what you will be limited to if you want to work with everyone.
For Back position workouts, you can do running. Pretty much any ladder or competition exercise is good. A basic distance + a sprint at the end is best.
For Forwards, lifting would be the best bet, but you’d be limited to shoulder, leg and core.
If you wanted to get some fun in and had a few healthy clients, teaching people how to do lineouts is fun. If you have two strong and about the same height, they can lift another person in the air to catch a ball.