My own son like plenty of down times, and seems to hate being scheduled at all, let alone overscheduled,
We are in the same category. My son needs time for himself and complains when he doesn't have enough of it. He usually pursues his own interest in his free time and learns a lot this way. Less seems to be more in his case. Lots of free time is one of the major reasons why he is in half day K and why we are considering homeschooling for next year.
If her music teacher thinks that she is ready for an instrument and she is interested then I would probably try it, but I would also make sure she has time for it (perhaps less time in Montessori). JMHO.
