Are you doing circuits with various pieces of equipment, or just kettlebells, and what are your standard moves?
One idea to mix things up is to have a focus, or theme, for the workout. For example, you could focus on unilateral, or one sided movements, and incorporate kettlebell exercises geared toward that (example is one legged deadlift or one arm chest press with kettlebell). Another theme could be leg and cardio drills with kettlebells. You get the idea—sometimes a focus or theme makes the circuit new and interesting for the client even though some of the moves will be the same.
You could also take some of your same moves and make them harder by varying your time (both timing in seconds as well as varying the rate of speed for some exercises). Or increase the weight of the kettlebell and do fewer reps.
Hope some of these simple suggestions add a little variety for you.