I wouldn't worry about teenagers finding something to do. Excursions are always fun, even when it rains and your out in it. Yes, there will be other teens aboard, but not as many as you'll see during June, July, and August. Yes, there is a basketball court on the Spirit, as well as table tennis tables. Yes, you can eat together at nearby tables. Most tables, if not all, aboard the Spirit sit 8 or less, so you'll find it difficult to get all of them at one table.
Have a great cruise.....