This is the thread where I asked a similar question before our first visit:
//ambergriscaye.com/forum/ubbthreads.php/topics/90754/1.html
We took five or six boxes of library supplies on our first visit, as well as a cord needed for the only printer the library had at the time. It had been donated but never used due to the absent cord. We ended up selecting that as our "cause" and have continued to take supplies to them on every trip. (I think I'm at about 17 trips now.)
We picked the library partly because, as Pam noted, it serves all of the children (and adults) on the island; but probably even more because of the very personal connection we feel to the public library system. Both of us grew up extremely poor and learned to love reading at a young age, as it was one of the very few "free" forms of entertainment. We both feel that love of reading eventually had a positive impact on our lives in ways that few other things could have.
Everyone has their favorite "cause" and all are worthwhile for different reasons. Good luck on finding yours!
