The reason preparing for medical school is a bad idea is not because you can't learn the material outside of medical school, its because the material you learn during the first two years of medical school is disproportionately the kind of useless minutiae that no one retains. I had biochem in undergrad, and it wasn't all that different in medical school, but it also wasn't easier in medical school because I had lost most of the knowledge waiting for med school to start. You won't remember the insertions of the muscles of the hand, or the finer points of the Krebs cycle, because the brain is not designed to retain information it has no use for or emotional connection with. If you learn it you'll forget it. Then you'll take it in medical school and forget it all again.
Don't waste your time pre-learning a bunch of nonsense that you shouldn't really need to learn once. Its OK just to focus on hobbies outside of medicine. If you do insist on learning something prior to medical school, then focus on something that physicians need but either aren't taught or are taught badly. Some recommendations:
1) Languages. If you're not fluent in Spanish and you're trying to prep for medical school drop all your other plans and start learning Spanish. Translators are a myth made up by the liberal media, at least while you're a resident. You need to know Spanish or you will be doing veterinary medicine on a large portion of you patients. Travel abroad and go to language school if you have the money (
www.ecela.com). If you have more human finances Skype languages exchanges, rosetta stone, and the Spanish version of Harry Potter will usually do the trick.
2) Learn personal finance. Seriously: start with the millionaire next door and the boggleheads guide to investing. Then move on to white coat investor's book, and read through his recommended reading list. If you want to take a course, take on in accounting.
3) If you really want more classes, take them in subjects that medical schools should teach, but don't. Nutrition would top my list. Statistics is also good.
4) If you're really long on free time and completely out of ideas, get involved with clinical research.