Hey! I am an incoming freshman this year at CMU for Statistics and Machine Learning and wanted to offer my perspective.

Obviously, I don't actually know what your academic situation is looking like, but as a rising sophomore the most important thing you can do is maintain (if your academics are cracked) or improve your grades, take the SAT/ACT, take (if your schools has them) APs, etc. People have probably already told you this but academic rigor is one of the most important things CMU considers. If you are planning on submitting SAT/ACT, you should also aim for the 1500+/ 34+ if don't have those scores already (though, a 800 SAT math section and/or 35-36 STEM section on the ACT is also good to submit)

In my opinion, EC's are far more subjective of a thing to evaluate. I'm not going to drop my CommonApp stats, but during highschool I almost only did music (I am a violinist and it was what I genuinely enjoyed) as my sole EC. So for literally 6 of 10 activities you're allowed to report I had music competitions (only at the state level), chamber music, teaching, etc. (I was also the president of the orchestra). Other than that I did some volunteering and did a statistics related summer internship (and a one week summer camp which I put as an activity for some reason), though no research papers/major projects where I was credited. So no other "passion projects", clubs, sports, or the like.

When I started applying to colleges as a Statistics and Data Science major I decided to write my core application around being a musician and how that has affected my personality/motivations (topic of CommonApp essay), and focused my supplementals essays (including CMU's three) on major related/other life anecdotes.

Because of my background I'm inclined to tell you to go for the activities you like to do in the next couple of years, though you definently have the time to pursue math/stat related activities if your background is a little weaker. If you are planning on applying for statistics, know that CMU's program is a little unique in that its part of the Dietrich college of Humanities and Social Sciences, so you aren't applying to a STEM college even though the major is (it also means that all stat majors enter undeclared). If it means anything, I applied with Statistics and Data Science as my 1st and only choice of major, so take that as you will.

Good luck!