Your post implies that you are a rising senior, so I'm giving advice based on that. Your GPA and extracurriculars are pretty much already set in stone, although I would shoot for leadership positions in athletics and student council this coming year. All this being said, there are essentially three areas that you can improve in, standardized testing, fitness scores, and the interview.
For standardized testing, there are plenty of resources online like Kahn academy. You would be amazed by how much you will improve if you practice for 30 minutes a day the entire summer. If you can get your ACT score into the 30s, it will definitely help make up for a lower than average GPA.
As for fitness, start practicing for it now. Look up the test for the branch you want to join and train like a maniac. Go out sometime in the next week, and run a full practice fitness test on your own, making sure your form/timing is in line with the prescribed regulations. Then, build out a fitness plan, and stick to it, you will start seeing results in under a month.
For there interview, get an adult you know to meet you and practice the interview. Although practicing with someone from a military background would be ideal, any professional individual would do. Treat it like the real thing, have them ask you the hard questions, don't use any filler words, and dress like a professional.