First of all, you’re doing great currently - just keep it up! Don’t do activities that you don’t want to do, only in an attempt to “buff” your applications. Do what you love. A summer STEM program or two? A rock climbing camp? Sure.
If your artwork is considered to be exceptional, you might want to look into the Young Arts competitions. Apply to the National Arts Competition | YoungArts
The reality is that you’re surely going to get into one of the public California U’s, for a bargain education at in-state tuition rates. You might get into one of the tippy-top schools you’ve mentioned, because you’re a great STEM student, and being female will help. Taking multivariable calc and both semesters of Physics C will help, too. If you qualify as an underrepresented minority (assuming that the Supreme Court hasn’t ruled against this by the time your class is applying to college), you really would have a decent shot at even the absolute top schools for STEM, including MIT.
So no, you don’t have to do anything differently from what you’re currently doing. Certainly, summer STEM programs this summer will help. So will any other programs in which you take a leadership role, so being a coach/counselor for younger kids will help. Continue submitting your artwork to competitions - an award would look nice. Also, if you want schools outside of the UC system, you should prep for and take either the SAT or ACT, so that you can submit a high standardized test score, in addition to your PSAT.
Begin the financials conversation with your parents. If you’re low income, or even low-middle income, you could get a full ride at a tippy-top school which meets full need. That means that expensive private schools could wind up costing you LESS than a UC. If you’re upper middle income, then the UCs might be your best bet, since you could be deemed full or near full pay at a private school, but your parents might not be able to afford what the school thinks they should pay. If your parents are high income or have rental real estate or business assets, you would be deemed full pay - but your parents might not want to pay over 360K for college for you! So in addition to keeping up the excellent work you are doing, it’s time to talk finances with them.