What’s your take on changing protocols to maintain client interest? If switching now and then to a less efficient but valid scheme keeps a trainee enthusiastic and making some progress, I’d think he’s better off than if he quit a “perfect” program, (assuming safety isn’t compromised).
The exercise protocol you use with a client should be determined by and if necessary modified or changed based on their goals and ability, not what they find interesting. What they should be interested in is results.
Because so many people confuse exercise with physical recreation, the distinction between the two is one of the first things you should explain to new clients. The purpose of exercise is to stimulate the body to improve or maintain functional ability, health, and physical appearance and the requirements for doing so dictate the protocol, not what a person finds fun or entertaining. By trying to make exercise fun or trying to turn a recreational activity into exercise you end up with something that is not very good for either; an activity that is relatively inefficient and ineffective for stimulating physical improvements and not as much fun.
I tell new clients that as they become better at performing the exercises their workouts will become more challenging, and to expect them to be very hard. They should expect their muscles to burn, their heart to race, and their breathing to become labored, and that their workouts will not be fun – but they will be mercifully brief and infrequent, and they will be very effective and very safe.
Rather than compromise their workouts encourage them to participate in physical recreational activities and those will give them additional motivation to do well during their workouts; the more their functional ability improves as a result of exercise the more enjoyment they will derive from other physical activities and the more resistant they will be to injury when performing them.
This is not to say you should never vary someone’s program, but any change in protocol, exercise selection, volume, frequency, etc. should be done for the purpose of improving the effectiveness and safety of the program based on their goals and response to exercise, and not arbitrary or for the sake of entertaining them. For more on this read The Ultimate Routine.