My credentials in high school:
GPA: Unweighted 3.7, Weighted 4.4
At the time of my application my extracurricular activities were as follows:
1) Piano: 13 years (With several competition wins)
2) Violin: 8 years
3) Mock Trial: 4 years (With county and state wins)
4) Capital Markets Contest (1st place)
5) Tennis: 13 years (4 years on HS team)
6) Community service: 203 hours
Really it's consistency that counts, but winning competitions doesn't hurt either. I applied to 3 schools: UCLA, UC Berkeley, and Yale. I only got into Cal, so I had no choice but to go here.
For you to get in based on my understanding and what I've witnessed from others at the university:
1) Academics don't hurt to be good, but as I am an example (and I know others as well) you can get in with mediocre grades and a bad SAT/ACT score, so even if you get low SAT/ACT score or have a bad GPA, still apply, you may get in.
2) Extra-curricular activities and consistency are vital. Cal wants to see
A) You are involved in stuff outside school and
B) You are consistent in your outside endeavors.
I got in over other students at my high school who had better grades and better SAT/ACT scores than me. Hope this helps in your application process and your choices in high school.
In the long run, you should understand that SAT/ACT is subjective and does not prove if a student is "smart". Cal admissions is aware of this, and you better show that you can be a beneficial citizen in society.