You need to stretch your mind from time to time, make it do things that it doesn't ordinarily do. What I do is listen to a podcast in Italian, or write a blog post in bad French, or read a paper about linguistics. Thinking, teaching, or writing about your own sub specialty does not count as stretching. Reading for pure entertainment is fine, but it is not the kind of stretching I am talking about. When you are stretching, you should feel slightly uncomfortable. Listening to familiar music is not stretching, but trying to follow along in the the score while you are listening is (if you are not a good musician.) When you are stretching, you should feel your mind-muscles going someplace where they are not used to going. It should feel like an actual stretch.
The purpose of the stretch is to remind yourself that what might come easy to you might be difficult for someone else; to increase and maintain mental agility; to have some fun; to learn something new and possibly to get some ideas that wouldn't have otherwise occurred to you.