There are plenty of bookstores on MV: some specialize in local authors, while others collect antique publications. Music stores are a little more sparse, but it is possible to find a radio, a guitar, or a set of bongos if you know where to look.
No room left in your suitcase? Visit one of the Vineyard’s six public libraries and borrow a book, CD, or movie.
If you’re artistically inclined (or even if you just like to color) there are plenty of other stores where you can pick up supplies, games, and more.