It's hot going, but the main roads are well-surfaced, but watch out for bad driving. Ask at the ubiquitous scooter rental shops about bike rental.
The island has extensive boat links to Ko Samui, Ko Pha-Ngan, Ko Tao and Donsak Pier (for Surat Thani); it's the only way to get around between the islands.
The closest thing to a bus on the island is a sŏrng·tăa·ou (pick-up minibus) which race around the island's ring road.
Car & Motorcycle
You can rent motorcycles and scooters from almost every resort on the island. The going rate for a scooter is 150B to 200B per day, but for longer periods try to negotiate a better rate. You'll generally need to hand over your passport as a deposit (but you may need it if moving hotel, so plan ahead). You'll see petrol for sales in bottles at 40B at litre, but it's much cheaper at petrol stations.
If you decide to rent a motorcycle, protect yourself by wearing a helmet, and ask for one with a plastic visor; keep to driving slowly. If you rent a motorbike or scooter, be warned that if you don't have an international driving licence, you may have problems in the event of an accident and your insurer might not cover you. You won't be asked for one when you rent, but the renter won't be interested in what happens to you on that front, as long as damage to the motorbike is paid for.
Even if you escape unscathed from a motorbike riding experience, some shops will claim that you damaged your rental and will try to extort some serious cash. The best way to avoid this is to take copious photos of your vehicle (cars included) at the time of rental, making sure the person renting you the vehicle sees you do it (they will be less likely to make false claims against you if they know you have photos).
If they still make a claim against you, keep your cool. Losing your temper won't help you win the argument and could significantly escalate the problem.
If things get really bad call the tourist police, not the regular police.
Drivers of sŏrng·tăa·ou (pick-up minibuses) love to try to overcharge you, so it’s always best to ask a third party for current rates, as they can change with the season. These vehicles run regularly during daylight hours. It’s about 50B to travel between beaches, and no more than 100B to travel halfway across the island. Figure about 20B for a five-minute ride on a motorcycle taxi.
There is no railway on Ko Samui, Ko Pha-Ngan or Ko Tao. The nearest train links are in Surat Thani and Chumphon.
Taxi service is quite chaotic due to the plethora of cabs. In the past taxi fares were unwieldy; these days prices are more standardised across the islands (though fares are still ridiculously inflated compared to Bangkok). Taxis typically charge around 500B for an airport transfer. Some Hat Chaweng travel agencies can arrange minibus taxis for less.