As you are in California, keep in mind that UCs don’t use your freshman year grades when calculating your GPA. They do look at those grades, but they care far more about your sophomore and junior years grades so, if you are interested in UC, that’s good news for you. Also, since you are in LA and interested in STEM, I will mention Amazon AHEAD x TKS, if you happen to be part of a group that has been historically underrepresented in STEM (defined as Black, Latinx, and Native American students and those that identify as female). It is actually a pretty amazing opportunity for STEM students - even my daughter, who has really never been a STEM girl, has really enjoyed the work. You do a number of projects in the program - all of which are potentially great fodder for your college essays.
Another possibility is taking dual enrollment courses at your local community college over summer. You can usually take up to three and in many places those courses are free for high school students and there are a ton of options. They are also transferable credits for UC/CSUs.