More than half the roads on the far side of Koh Tao are unpaved. Just miles and miles of unpaved, untrafficked, and rugged terrain, just waiting to be raced over with a tough mountain bike. There are several places on the island where you can rent a mountain bike starting at just 150b per day.
For a bit more juice, try the same trails on a 250cc dirt bike. Reach some of the most inaccessible places on the island and give yourself a thrill while doing it.