Getting around Koh Tao is quite easy when you factor in all the different possibilities. Beginning with taxis, most destinations around the island will cost you about 350 to 450 THB. This becomes much more affordable as you share rides and divide the cost. When arriving at the pier, there will likely be a representetive from your reserved guesthouse to pick you up. If you haven't pre-booked, many touts will offer you free taxi to the guesthouse they are promoting. Taxi drivers are usually lingering in central Sairee, central Mae Haad, and central Chalok Ban Kao to transport visitors where they want to go.
Koh Tao is a small island with a limited amount of developed roads. Every year access around the island is improving, however there are still some beaches and areas that can only be accessed via 4 WD vehicle or through long tail boat. Taxis tend to be equiped for the terrain, as well as a taxi boat which will cost between 150 to 800 THB per person. A long tail boat can also be chartered for appx. 3000 THB a day depending on the amount of people sharing.
Walking and cycling through the island is another possibility. Be aware however that the roads are built on hills and combined with the heat and sun may make for a difficult journey.
Rental motorbikes and ATVs are becoming the most common mode of transportation through the island. A scooter costs as little as 150 THB per day and appx 500 THB for a small ATV. Prices do not iclude petrol and the rentals are always given with an empty tank. Remember that some Koh Tao roads can be dangerous for inexperienced drivers and caution should be used. Motorbike damages can be expensive, so drive only where you feel confident.