I had a client like this. I would ask for 15 of something and she wanted to do 25. The best way I found to train her was to move her quickly from exercise to exercise. I mixed it up with machines, body weight, and small tools (physio balls, med balls, free wts). I designed a program that was 1 set of 15 with multiple exercises. If we had MORE time I would have run through the program a second time. I found when I built in 3 min cardio bursts in between every few exercises, she was more about “what’s next” instead of I want more! It was a lot of fun although it took a ton of planning. I think over time we both really enjoyed it and she got great results. Every week I would intro a new exercise to replace the way or equipment she had used the previous session so it was fresh and exciting.