Only thing I saw when I was there back in the early 2000 was scooters for hire. The island is not that big considering half of it was/is US military war games area, don't know if that changed or not since then, but scooter would work fine. That is a rather sore spot about the island as it has been said they have a rather high carcinogenic rate from all the bombing on the island's other half. I had a rental car and left it on the mainland and hitched when I got to the island.
One thing I would not miss if your going to the island is to check out the Bio Bay, one of the neatest swims I've ever had. Freaky cool to light up like that. Like underwater fireflys to the extreme.