Your primary concern as a pre-med are getting the highest GPA and MCAT possible. I would use the study tactics that others have mentioned above to help improve your academic performance.
Another big part is ECs. Depending on what you want to get out of it, you can tailor them to be very efficient time-wise, yet still look good. Unless you are in desperate need for money, I would skip paid employment.
It's dime a dozen, and won't set you apart. Worst of all, it requires a significant time commitment. This can bite into study time, and leave a nasty impact on your grades and MCAT. This will sink your application.
The absolute best bang for your buck time-wise is clinical volunteering. It sucks to not get paid, but you're paying for the convenience. Checking the boxes for volunteering and clinical experience all while doing it for a few hours once per week simply can't be beat. If you find yourself spending a majority of your shift sitting around doing nothing, then take advantage of it and spend that time studying for classes. Boom! There's some extra study time right there! A lot of people meet physicians during their shifts, and end up shadowing doctors during their volunteer shifts. You can double count those hours. Boom! There's even more efficiency.
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As for non-clinical volunteering, the recent changes to AMCAS no longer ask you to put in hours per week. Therefore, you can "binge volunteer" sporadically at these volunteer sites, and rack up hours when it's most convenient for you. Since you don't have to put in hours per week, you can simply do a lot of hours all at the same time, and with longevity, it will be a very fine looking activity on your application. If you're not in summer school, summers are a great time to rack up those non-clinical hours.
I'm probably not the best person to ask about research. I only did research for one summer, and since I wasn't in summer school at the time, I didn't have to worry about studying. I'm guessing that with research, you may have some down time to do your schoolwork. And remember, unlike volunteering and clinical experience, research isn't necessarily a requirement for medical school. Unless you're applying to research institutions, you might consider skipping it in favor of better grades and MCAT, which would ultimately have a greater impact on your application.
Good luck!
