Congratulation on getting into both Summer Seminars!
Last year, I was fortunate enough to go to both. USAFA's Summer Seminar was more of an eye-opening experience that focused on the admission process itself with alot of briefs and classroom tours. At NASS, you will experience more of a cadet's life, where you wake up really early and do pt everymorning. Honestly, I really liked USNA's SS much better because it was very physical and felt like I went there for a week. But, USAFA's SS really helped me on what to do right when I went home for the admission process.
One advice I would give is to prepare for the CFA, if you have already started. You will get to do the official CFA at USNA. If you get a score you like, then you already finished one part of your application. But most importantly, if I could go again, I would have went with being less serious about it. Do not stress over the Seminars. Go there with an open mind and just have fun.
Recap: work on your push ups, pull ups, and mile run; and just go have fun when you get there
Hopefully, this helped and good luck on your future application process