I find it ironic how frequently posts like this ask for advice from people who've "already gone through it" yet reject the near-unanimous advice from those people to not study at all.
If you insist that you want to spend your free time preparing for a medical career, I would, in order of priority:
1) Either learn medical Spanish or develop a new hobby you're interested in and can do in med school. There is a very short window of time when people will care at all what you scored on boards. They'll always care about who you are as a person.
2) Shadow a diverse group of fields, even stuff you aren't as interested in. I can't quote the source, but anecdotally I've been told 80% of med students go into a different specialty than they initially thought they would.
3) If you have to study, I'd study anatomy, and maybe a little histology. Being able to quickly identify a histology slide - while something you'll probably never actually do in real life - is something you'll always be tested on, including potentially on your medical boards. While having a good knowledge base of biochem is nice, realistically that'll get you the first few weeks of med school at best.