The only reason to get another degree is if you need it to do what you want to do or if it's going to substantially increase your income. Or if you just have money you can throw away (which is fine--I'm all for enriching oneself).
I don't see any gain from doing and SMP or masters in biomedical science (which is pretty much the same thing as an SMP--a degree that really doesn't help increase your marketability and just costs you money). Neither degree is going to help you get a job, increase your odds of getting into medical school, etc. They're both there for the sole purpose of increasing your GPA to get into medical school (and/or to make the school money), and your GPA is already good. The MPH could potentially turn into a career so if you're really interested in public health then you could make an argument for it, but I don't think it'll make you competitive of an applicant compared to someone that worked/volunteered full time.
If you really want to go back to school and spend money, do something more useful, like learn Spanish. Talk about a great way to spend a gap year--go live in Central or South America for a year or two. Do volunteer work and learn the language through immersion--you have no idea how useful it will be (and it WILL increase your marketability for medical school and a career if you're fluent in Spanish). Consider the Peace Corps, Jesuit Volunteer Corps, etc. Need more action? Join the Army, Coast Guard, etc.--that will give you unique life experiences (experiences you often can't get later in life, due to family obligations, etc.)
If you're going to take time off and your GPA doesn't need work (which it doesn't), and you aren't interested in the above options, then I'd recommend working and continuing volunteering gigs. I took a gap year (two actually). The first year I TA'd physics for my post-bac program, took one course a semester for free due to TA'ing benefits, worked another 34hrs a week (more social-work related job), and kept up my 10hrs/week of volunteering. The following year I worked full time. I'm glad I worked instead of paying for another degree, but I think it would've been great to have been able to do any of what I mentioned above-- it would've made for a really great experience.
Bottom line is do something of value/meaning with your gap year.
