Studying formal logic never hurts, also. You can get a healthy dose of it from several common classes:

1) Discrete Math (Major level math class; does formal logic and also set theory, basic proof techniques, etc.)

2) Basic computer science class (virtually all introductory computer science classes will do some formal logic. It's not as rigorous as discrete math, but it gives you some intro to it)

3) Logic (philosphy major level class). Pretty straightforward here.

You could also study logic on your own. In any case, you don't need to be a master at formal logic, but it can definitely be helpful in certain cases, especially on the logical reasoning section but also commonly during games.

I only list the classes in case any of these overlap with your interests, majors, or minors.

Honestly, as the above poster noted, a diganostic couldn't hurt, but you have so much time it might not be very helpful. Personally, I wouldn't do one, but that's me.

Really, though, your main focus should be the highest possible GPA. Reading advanced material and studying formal logic are just some things that may help you maintain/strengthen basic LSAT skills.