One of the larger among many islands off the San Pablo River in Jacksonville, Castaway Island has a small boardwalk and a series of nature trails that take you just a little of the way into the isle's swampy recesses, but for the most part you're on your own. I happened by there yesterday and lingered awhile to soak up some tranquility.
There's a bike trail running from its entrance (which also connect to a greater network of them) to the nature center complex. At the end of the boardwalk, there's a tiny dock from which to launch your kayak or canoe. I chatted with another pilgrim who was hanging out there and asked him if there was much fishing here; he replied emphatically yes, it's a prime fishing spot.
There's a nice park which is directly adjacent to the island but not actually on it, which confused me a bit at first. Sometimes it's hard to tell that you've actually crossed over onto an island without consulting a map. And apparently, when the swampwater is low, the island technically becomes a peninsula until the creek rises.