I reocmmend looking into “Youth for Understanding” for a HS study abroad program
It sounds like the OP has some programs in mind. There are many church-sponsored programs (and the American Friends Service). WOOF’ing might be an option, or an exchange program. I know kids who worked in orphanages abroad, but don’t know the age restriction. In this country, Frontier Nursing is an interesting program. Look into NOLS too.
There is no need to continue academic studies during this gap year and also no benefit in increasing scores if they are already high (720+)). I don’t see the problem here.
Colleges, including elite schools, will be very interested in you after doing this, but wait to apply until after you have done your year abroad, not before- in other words, apply by Dec 31 2017 for the school year 2018-19.
Admissions at selective schools is all about what you bring to the mix of a class, and I think your experiences will offer a lot.
Good luck!