Pickle, Seawings, and others have brought up many valid points. First thing that should be paramount is: "Do JrROTC, CAP, Sea Cadets, or WHATEVER you do...... because you WANT TO. Because you have some PASSION for it. If you have passion; you'll most likely excel in it.
If you do any of these things because you think it's going to give you some leg up on an academy application; then you are quite wrong. Being in JrROTC, Cap, Scouts, etc... is not going to give you any more "Brownie Points" compared to being in 4H, FBLA, aeronautics club, DECA, Junior Statesmen of America, Orchestra, Marching band, etc.... What matters in ALL of your extra curricular activities, is that you demonstrate "passion, growth, leadership development, excelling, etc...". Same with volunteering time to the community, to the needy, hurricane victims, etc... You need to demonstrate your desire to excel in the activities and take on more responsibilities.
But as everyone pretty much has mentioned, your life has to have balance. You have to be well rounded. Yes, some individuals get truly consumed in whatever activity they have passion for. But as Seawings mentioned; they were able to do JrROTC and still have time for part time job, President of the NHS, and maintain good grades. This has to do with ANY activity you're into.
Of course, 2 areas that are extremely important are academics; "including good gpa, high class ranking, and excellent ACT/SAT scores"; and athletics. This includes not just being on an organized and competitive athletic team, but excelling in it and hopefully taking on and developing leadership roles and skills.
So again; join JrROTC, CAP, Scouts, or WHATEVER because you want to. If you're doing it because you believe it's going to give you some edge or leg up on your academy application, you're incorrect. Whoever told you it would, is wrong. All those activities I mentioned earlier in this post; none of them gets you "More Points" over another. The only activity that will get you "more" points, or I rather should say "LESS POINTS", is if you aren't involved in some form of organized/competitive sports/athletics. FWIW: I'm all for JrROTC, CAP, and scouting. I loved doing many of those things growing up. I just don't want you to try and equate:
"JrROTC, CAP, Scouts, etc... wear uniforms, take orders, have a pseudo military form of structure, etc... + I want to attend a military academy and become an officer; thus = I have a better chance of receiving an academy appointment if I have one of these pseudo military activities on my application".
You don't. Truth is; if you're passionate about the marching band or similar, and have done that for many years, and have worked your way up section leader, drum major, or whatever the leadership role is of that activity; you'll have more points on your application than a person in JrROTC, CAP, etc... who simply is "Part of it" and doesn't have the leadership roles in that activity. This is the same with ALL activities. "MEMBERSHIP" is nice. "LEADERSHIP" is desirable. So, be a part of whatever you have a passion for. The list of activities you can get into is almost endless; and NONE are given more points than others. Leadership gets more points than membership however. Best of luck.