Renting a motorcycle in Thailand

Don't do it. The traffic can be heavy on those twisty island roads. The vehicle maintenance can be poor. The accident rate for novice bike renters in Thailand is very high and can result in life changing injuries or death. And your insurance company will almost certainly try to get out of any coverage if your license in invalid, or you rented from someone who was happy to ignore the law.

Rent a jeep instead. I did that in Ko Samui and had a nice day out. Just make sure you check the vehicle carefully, tyres, engine, lights, bodywork, fittings - 500m down the road in the first jeep I got, I found the drivers seat wasn't properly fixed to the floor, so I took it back and insisted they changed it for a different one.

One last thing: don't hand over your passport as security. A common scam is to claim some minor but expensive damage, and them holding onto your passport until you pay up.

....is it a terrible idea to rent a scooter over there?

Hard to be objective with a question like this, but I suppose some experience would help you along.

If you drive the automatic scooters that are 110-125ccs (Honda Scoopy, Yamaha Fino/Filano, Honda Moove, Honda click, Yamaha Nouvo, etc....) within the last 10 model years or so^, and

• you drive at a speed that is compatible with road conditions (generally 40-60km/h)
• stay to the left when in doubt
• keep your head on a swivel

You should be fine. Practice on local side streets first before going into crowded areas. In fact, I would recommend FIRST using a motorbike taxi a few times, then ride with a local friend 2ND, then ride by yourself.

@Gnusper has a point in that Westerners tend to be neurotic. But you will see Thai-style driving is NOT the same because the thinking and safety considerations for self and others are structurally different. #1 reason I see tourist who get into motorbike accidents here are because they try to drive like a Westerner in Thailand.

It's unlikely that you will get travel insurance that will properly cover motorcycle driving in Thailand. Don't be surprised if in your search the insurance contract specifically excludes Thailand or has ridiculous restrictions on riding motorcycles. If you do find it, please leave it in the comments or update your answer.

I hope I didn't scare you. Motorbike is a [relatively] cheap and convenient way to move around quickly. You will have lots of fun and most tourist will never have a serious incident or notice anything too out of the ordinary during their month or so stay. Stay long enough (start growing some roots like having children) you will begin to lose some of your innocence.
_{^If you are over 125kg, you could opt for 150cc...anything over that for an inexperienced rider is likely overkill.}

You are sure you are allowed to drive motorcycles up to 125cc in every country? According to EU law and given your information that you never sit on a motorcycle before, you are only allowed to drive motorcycles in Spain (driver's license guideline article 6, number 3b - here in German ). So far from the legal point of view - you should consider this especially in the terms of insurance (medical and motorcycle insurance as well). You might also consider to get a scooter according to the EU AM driver's license - scooter up to 45 km/h. This would be legal.

However, everyone goes with a motorcycle in Thailand. You should not have problems getting a 125cc machine if you want to. Regarding the "experience" question - I had the same problem like you. After driving 10 minutes in a rural by-road I had no problems handling my bike. Maybe you should avoid crowded areas at the beginning. But if you are experienced with bicycles you should handle an AM scooter or even a 125cc motorcycle.