Your options are to either enroll as an extension student and take regular classes or look at other universities in CA, such as the CSU. Not all CSUs accept second bachelor students but a good number of them do, especially if you are going to pursue a major that is not impacted.

As far as I know, almost all universities, even private ones, let you take classes as a non-degree student. If you just want to earn some credit before applying to graduate schools, this may be the way to go. Also many of the CSU graduate programs have conditional admission where they give you a chance to make up the deficiency courses as a graduate student.

If your goal is to just raise your GPA, whatever you have graduate with is set in stone. However, you should ask the graduate programs you are interested in about how they deal with post-baccalaureate grades because some of the may just count the last 90 units.