Is There a Duty of Self-Improvement?

We often think of ethical duties in negative terms, as a series of thou shalt not rules. And while it certainly is true that in general, one ought not lie, one ought not steal, one ought not put glue sticks where your roommate usually keeps his roll-on deodorant, there are also positive duties, things you need to do. But where there are the ones that everyone has to do whenever called upon, what about the ones that you have a choice about?

It is a good thing to learn to speak another language, to pick up an instrument, to read a book that opens your mind to new horizons. These sort of activities enrich you as a human being. They make you more interesting, more well rounded, closer to the ideal self you could be. Do you have to do something like this? If someone stays out of trouble, but uses their leisure time to be a slug, never going to concerts, plays, museums, or lectures, never finds films or books that make him rethink his assumptions about the world, never works on his backhand or gets in shape, is there something morally wrong with the lack of action? Do we have a duty to -- not at all times, but often enough -- do something to grow?