A previous post suggested that there was no hiking possible on Rattlesnake Island. I believe that it is in fact possible for non-residents of Rattlesnake Island to hike there. The Lakes Region Conservation Trust owns about 100 acres on Rattlesnake Island (called the Morris and Pardoe gift on their website - link below). You would have to call them to confirm how to access this but I believe they have a dock on the southern end of the island with a marked trail that will take you to the top of the island. It is listed on their website below as a "publicly accessible property" along with Stonedam Island, Five Mile Island and some others. I would recommend calling them so that you understand whatever rules they have about property access. Their contact info is on their website.
http://www.lrct.org/lake-winnipesaukee.html