I've not stayed on the island, but here's a short report which may help-

To get to Koh Khao, head north from Khao Lak towards Takuapa and turn left after the #82 kilometre stone. If you get to #83 - you went too far. Follow a somewhat winding main road to the ferry area. The ferry runs from about 7am to 6pm although recent reports suggest it runs as a private rather than public ferry after these times. If you are on a motorbike there are long tail boats at the dock that will take you across the bay.
The ferry costs around B150 for cars (B300 after 6 pm) and B20 for motorbikes - but the prices seem to vary from time to time so be prepared to pay a bit more (probably in high season) and be happy if you get to pay a bit less.
Once on the island there is a long road that runs north to south - with little traffic - and you can access the full length of the island from this road. Head down the roads that go west from that road and you will generally head to beaches and one of the resorts in the area. Those roads are usually paved. The roads to the right head into the countryside and small villages and are usually dirt and can be rough.
Take the road to the tip of the island and you will find a good vantage point and nice little beach restaurant. A walk up the beach will allow a good view of the next uninhabited island. There is a sign just before you reach there for another restaurant facing this island but you need a 4X4 to get through the deep sand to it.
A few years ago there were only two restaurants outside the handful of resorts on the island. Now there are a growing number at the ferry dock, on the road leading up the island and, inevitably, near the entrance to the major resorts, and typical shacks on the beach also.
Keep your eyes open for interesting bird life on the power lines - lots of kingfishers especially - and iguanas sunning themselves on the road. Or vice versa!
Drive to the far north of the island to find an old Japanese airstrip - used during WWII - that is scheduled to become a local airport.
Take your time and enjoy the solitude of this area! Consider staying on the island some time, if you don't mind a very relaxed and quiet time. You'll find resort prices here to usually be less than in the Khao Lak area.