I need more extracurriculars!!

Personal opinion, if you're already doing a lot and in Junior year, it would be way more beneficial to your application if you just went more in-depth with the stuff you're already doing! To join a new club or learn a new skill this late in the game wouldn't be super helpful, because you probably wouldn't be able to really hone your skills in whatever it is you're trying to do- for example, joining the soccer team but not really knowing much about soccer and not winning any games or going to tournaments. I think the goal isn't to try to 'fluff up' your application with more activities, but to show dedication to projects and organizations in your community and outside of school? (Edit: spelling)

The easiest option for you in my opinion, considering how much you have already accomplished and plan on doing, is to really just go into specifics and show how hard you've worked. Being involved with extra curriculars is a good idea but colleges are more likely going to focus on the ones that you really describe. You would have more of a good stance if you went into really describing this volunteering opportunity and talking about the local nonprofit, than joining let's say art club or cheerleading and not really have much to say about it, nor show for it. The important thing is, to make yourself look as good as possible without cluttering up your application and experiences. Hope that helps!