I could not disagree more with Zilber about the need for flight time prior to flight school. I started API with zero flight time and a degree in political science, and I did just fine. Dropping 10 grand on an instrument rating right before the Navy pays you to get one does not make a hell of a lot of sense to me. I would guess that less than 5% of SNA's start primary with flight time other than what the Navy gives them in IFS. Guys pick up their first choice every week who did not have an instrument rating before flight school. IFR procedures are pretty cut and dry, and if it takes you $10,000 worth of prior instruction just to get through flight school, you are are really going to be hurting later on when you get to weapons, tactics, and all the other complex stuff in your fleet aircraft.
As for classes that you can take in your last semester, it depends on your situation. If you have already been accepted for SNA, take it easy. If you are still in the application process, taking calculus or physics would be a good idea. All ROTC students are required to take 2 semesters of each no matter what their major. If you are going to have a good package, you need to have a good GPA and electives that make it look like you actually took college seriously.