You don't need a motorbike to tour the island, it's just one short paved road and then it's beach front resorts. First the bad, it's all overpriced and packaged for tourism. Ahh... but it was not always. The first thing they do when you arrive is load up all the farangs in a songtheaw and head to the entrance to the National Park where some official collects a 400B park entrance fee. Where's the Park? It has not been a real National Park since 1990. If you're Thai or have a Thai DL you pay 40B (maybe) but most visiting Thais don't even do that, just don't get in the truck that is there to collect the fee. Not that I am suggesting that you avoid paying the outrageous fee, it's probably a crime to do so.
Now, the good. The water and beach are very nice. The food at the smaller restaurants is good and not expensive, just stay away from any restaurant run by a hotel. The hotels do this trendy beach table dinner and entertainment aka the fire show. Sit on a mat, swat the flies, ticks and sand fleas. That's one thing that has improved, it used to be infested with mossies but now it's been under control, I don't know what they did, massive amounts of insecticide?
If you do go drop in Richard's bar, if it's still there and he's still alive. He is or was (if now dead) a long time resident of Kho Samet. He has old photos of the place back in the 80's. He's from Finland and is quite a character.
Personally, I prefer Kho Chang over Kho Samet. It's less touristy and there's some quiet natural places away from Whitesands beach.
I used to frequent Kho Samet in the late 80's, early 90's and it was a nice natural National Park. There were many deserted coves and undeveloped sea front. Bungalows were the only lodging and nothing over 600B per night. Now, I have returned a few time to see the island get gutted and packaged for development. I don't go any longer.
The water and beach are just as nice at Kho Larn, if you only have a limited amount of time consider staying one night there or just a day trip.