The largest uninhabited island in Tokyo Bay, located in Kanagawa Prefecture

Although no monkeys inhabit it, it's called Sarushima (Monkey Island). It's the largest island in Tokyo Bay, but overnight stays and camping are not permitted. It gained attention for resembling the fortress in Laputa: Castle in the Sky, but even setting that aside, the wartime remnants scattered around the island are impressive and make you stand up straight. Just a heads up: the island's only shop gets insanely crowded, walking around the entire island takes less than an hour, and the beach is just average (middle of the road), so don't get your hopes up too high lol.

Enoshima