Academics: Try to get strong exposure to English (AP lang/lit), Computer Science, Chemistry, and Physics. This will give you a strong foundation and placement at USAFA. That way you will do well wherever you are placed, and possibly advanced so you can get to higher level/major specific classes faster. Work backwards from senior year and map out your academic course strategy.
Woulda/should/coulda from my DD:
Woulda taken MICRO econ to place out of entry econ class; MACRO actually places you out of higher level class and probably only used if you major in Econ (she wants to double major in Econ + one other)
Woulda tried harder in History AP to get a 5 a place out of Hist 300
Glad she still took Art Hist as it was interesting and actually helped in Eng 211 subject matter
Glad she took AP French and placed out
Glad she took Phys 1 and C as one of her life goals was to avoid taking any Phys at USAFA. Unfortunately, she now wishes she took Phys C/M to place out of Phys 215 which is a gateway class, and if you are interested in the new Space Warfare minor, it is one of 2 required classes, and so...
Woulda taken Phys C/M instead of Envir Sci to place out of Phys 215
Wonderful that band and JROTC are actual classes! Try to achieve as high as you can, make a difference, state/national award levels, leadership etc. My DD was staff and PT club captain, and the latter was much more measureable in terms of who it served and her competition performance. Plus it was very compatible with training for the CFA. Sport - look at what's available, what works with your schedule, and when in doubt, track/cross country IMO are the best IF you truly have no preference but want to do a sport. It will help with your running plus is good overall exercise.
Starting this early - my best advice is to strategize your summers - rising Junior year make studying for SAT/ACT a priority - take several tests in the summer and early fall, and then you are DONE with standardized testing. You will also have great scores for SA summer program applications plus the spillover for strong PSAT score and NM finalist etc. RESERVE your rising Sr year for all the SA summer camps/Boys or Girls state and preparing for the CFA and getting as many elements done to trigger DoDMERB during the summer before your senior year starts.