If you want to experience Thassos the right way you need to have a car for at least two days. It would be ideal if everybody came with their own car but also coming with the bus has it's own advantages, especially if the bus is new and comfortable.
This year bus to Thassos costs 80e per person if you buy the ticket through some travel agency, which is a bit expensive. If you don't travel alone it is more affordable to come with the car.
On the island there are a lot of rent-a-car agencies. The prices are different and they depend of the demand. Agency Speedy in Limenaria is a bit more afordable than the others. When renting a car it is important to have an ensurance. The cars in the agency Speedy do not have ensured tires. Extra charge is 1e per day and it is not obligatory. Most of the other agencies have cars with ensured tires.
There are also some agencies that have just the basic insurance that doesn't cover the damage in case that you cause the accident. You have to be very careful when you rent the car and you have to try to get the confirmation that the car is insured in total, because some times the employees in the offices try to avoid that question and tend to tell you all the information after the accident happens.
If you take the car for one day than you have the limitation of 100km per day. Road around the island has 92km so I doubt that you will cross more kilometers than it is allowed and if you do the extra charge is not to high.
The price of the fuel is a bit more expesive than in Serbia.
International driving licence is not necessary. You need only green card for the Macedonia.