Unfortunately Feinstein is running against another Dem and probably won't be seriously challenged, but you can still send her a message by making it closer.  You also have Gavin Newsome to vote against in the Gubernatorial race.  Plus depending on what House district you live in, you may have a competitive race. And also ballot initiatives, etc.    With voting, depending on where you live, you sometimes have to take smaller local victories and take a long term approach.  There isn't always immediate gratification so don't get too down if your vote doesn't change things too radically in 2018.    This will be a process.