Getting your 80 units certainly won't prove difficult. You can get units for just about everything, such as taking sport classes. (Maybe it's only non-Americans who find this surprising.) But nevertheless, you have to enroll in at least 11 units a quarter to complete full-tuition quarters, and sometimes you happen to not want to take any courses for a quarter. Well, there are other options. You can get credit for whatever you wanted to do anyway by signing up with some faculty member for Directed Reading or Directed Research. You have to find a faculty member to do it with, and remember to fill in their code as the section number (i.e., you don't just enter 01 as usual---the Axess course-registration program will help you find the right code.) Some people think it is a good idea to do some Directed Reading early in your career. You can read about what is worrying you most at the moment, and you have a chance to interact more closely with a faculty member, perhaps checking out whether they'd make a good dissertation advisor.