Have the ability for clients to add a comment under each exercise (not just weights & reps). So, eg. client can write in "right shoulder was bothering me today" or "knee was not happy" or "next time, I should increase my weights"

This is something that was possible in QuickCoach since they did not have specific boxes for entering weights & reps so people could write whatever they wanted in the results box, and then see that result next time.