Kume-jima (久米島), the furthest flung of the Okinawan islands – located about 90km west of Naha, from port to port, as the crow flies – is close enough for easy access and just far enough away to retain that real 'island' feel. Its location and transport connections, while convenient and affordable, mean that the island isn't really a day-trip destination, unlike the neighbouring Keramas. The plus side to this is that Kume-jima rarely feels crowded.
While there are some mountainous areas offering spectacular views, the island's 63.5 sq km of terrain is mostly flat and covered with sugar cane. Its shoreline boasts a few lovely beaches and the mother of all sandbars off its east coast. There's also a decent selection of interesting historic sites, vistas and activities beyond snorkelling and diving, making Kumejima a worthy overnight or two-night stay.
For a moment at least, you might not want to leave.