John, if you live in a really humid environment, the easiest thing to do would be to get a room dehumidifier. My father uses a couple in the basement of his house in Boston where he stores all his books. You could protect your instruments and everything else from humidity and mold.
But if you are looking for something to put in your instrument case, look for something called a "two way" humidification system. That will humidify when the environment is dry and dehumidify when it is humid. I'm not sure exactly how (or even if) they work. Supposedly they keep humidity in the middle of the safe range, which is around 50 percent or so.
--
Dusty T., Northern California
Site Moderator
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie