The Greek island of Kos is generally somewhere that we wouldn’t advise visitors to get a hire car, and there are two main reasons for this. The first and the most obvious reason is that this is one of the smaller Greek islands, where all of the towns and villages are relatively close to each other, and there are plenty of buses and taxis available, including between the airport and Kos town.
The second reason is simply because Kos is so close to mainland Turkey, and because it is also so well-connected with other nearby Greek islands by ferry, that if you’re planning to visit this part of the world, we would far rather recommend hopping onwards and seeing multiple destinations, rather than sticking in just the one. This naturally leans towards visiting places by bus, and taking advantage of the much cheaper foot passenger ferry rates.
Needless to say, we would advise strongly against even trying to take a Greek hire car into mainland Turkey.
However, moving around between islands isn’t necessarily going to appeal to everyone, and a hire car might still be useful if you are staying in a villa or other resort that isn’t near any bus stop, although that again is going be down to how far you are prepared to walk and how long you are prepared to wait for a bus with no definite idea about when it’s going to turn up. For many, this is part of what Greek life is all about, so this really might not be a problem.