An archeological dig on Fox Island in 1995 uncovered fortifications from Queene Anne's War in the early 18th century. The lower half of Fox Island is a popular picnic spot, and locals say that you can still find cannon half-buried in the ground. I've never come across any, though.
You can park in the community of Champney's West, off route 230. There's a semi-improved trail to the top of Fox Island; you can check out the Discovery Trail website for details.
The steep climb up Fox Island requires some hands-and-feet scrambling, and probably isn't suitable for smaller kids. Medium sized kids and dogs, however, seem to like it plenty.
Left in the cache initially are a couple of small glow sticks, keychain Monopoly and Mousetrap games, and some other odds and ends.