Bodybuilding. That's what I was doing at your age. Think of yourself making a sculpture of something that's chics are going to dig. Say it in the Ahnold voice.
Im going to 10th next year and thats what im doing this summer. Even if you dont like sports and athletic things you might enjoy this (cause thats my case).
However theres other stuff I want to get into like web design but i dont really know what to do and where to learn.