By 'informal,' do you mean that you have your degree and want to take more classes without doing grad-school? I would definitely go for the upper-level science classes.
I did an informal post-bacc and would def. recommend it; just make sure you meet with an pre-health adviser to work out your schedule.
I re-took a couple courses I didn't do well in during my undergrad degree (C and C- classes), and then did some upper-level bio classes, research, and shadowing. My program was called the "Graduate-Student-at-Large" program, and from what I've seen most schools have it. If you go this route, I suggest you go to a higher-tiered academic school than where you started. And it was much cheaper than doing a formal post-bacc (about 1/4 the cost).
