NHS, Student Senate, a debate team for demonstrating leadership. Piano is great, but you can't march with a piano. Perhaps you could join the marching band and play the Marimba. My son is a pianist, and was recruited to play marimba and marching bells, rather than trumpet, for marching band. If you play piano for church, that counts for something as relates to volunteerism.
You could sign up for track, and make it your mission to excel at it. Don't drive to school, if you can help it. Walk, or run, depending upon how heavy your backpack is. I want to be encouraging, yet honest, so you realize what you are up against.
The truth is that if you are just beginning to engage in having a well-rounded life at the end of your junior year, it may appear that you are doing it just to look good on paper for a selection board. Have you been participating in classes at a health club, or the Y?
It's great to see that you still have academy aspirations, despite not being involved in sports. That shows that somehow you managed to stay away from the "wrong crowd" without being plugged into meaningful machismo team sports. Guess what? My son doesn't have a varsity letter. Yet, I have no doubt that this will not hurt him because of his other involvements/accomplishments.
If your lack of EC is justifiable (job to help the household income, coming from a single-parent family - and you have to manage household responsibilities - with younger sibs, or caring for ill parents/grandparents) the academies will take that into account. If your free-time has consisted of "hanging out" with your buds, goofin' off, strolling malls for chicks, or using what I call "electronic drugs" (video games), it's unlikely academy life would suit you well.
The academies expect well-honed time management skills. If you don't have them when you get there, I'm sure they will be happy to "help" you in your development. ;-P You will be spinning many plates at any academy. Academics, athletics, and military leadership, will be most of your day. Maintaining a spotless - shipshape room, reviewing the newspaper so you can discuss two world events during breakfast, memorizing your menus (for all three meals) for the day, and all other required memory work (military facts referred to as "required knowledge") will take up snippets of your free-time, if you can call that "free-time". You'll have a block of time each night for study time. After rising in the morning, you have your physical fitness, before morning meal formation.
Everything the academies do is for a reason. It doesn't have to make sense to us, for it to be a good reason. That includes the academy process. I know this response sounds harsh. Reality is that you will be competing with young men and women who have been building their resume for years. Hup to. You still have a chance if you get started tomorrow. If you do not live in a competitive district, even better. I hope you are up to the challenge.