I've been on both the NCL Gem and the Carnival Miracle - like comparing apples to oranges. Of the two, I liked the Gem much more. The Gem is bigger and has more amenities for kids. The Miracle is relatively small and while it has some good amenities, definitely lacking due to the size of the ship - definitely one of those things where bigger is better.
Your information is incorrect; it does have a basketball court onboard, at least it had one when I was on there.
But as for having fun with lots to do, it'll really depends on when you go, how many other teens are onboard, and the make-up of the group. They can be alot of fun or not - no one knows since each group is entirely different.
Hope you have a great cruise!