Many of the UC’s have Undeclared/Undecided exploratory programs but as @ucbalumnus stated that many STEM majors may be hard to change into later once you have decided on your major path. UCSC might be a good option since you are admitted into a “proposed major” and not required to declare a major until end of Sophomore year. Exception is Engineering majors in the Bourn College of Engineering.
Other UC Undeclared programs:
https://www.ucdavis.edu/majors/undeclared/
http://chassstudentaffairs.ucr.edu/undeclared_program/
http://cnasstudent.ucr.edu/undeclared.html