My husband and I plan to rent a car and drive from Dubrovnik to Kotor for a day trip in early October. We are from the U.S. and each have an international driver's license but have heard about the need for a "green card" which verifies insurance to the rental car agencies. We've also heard not all companies will let you take their cars across the border to Montenegro. Any ideas?
The rental company should provide you with the "green card" as part of the rental documents. It verifies insurance to foreign police forces (insurance equivalent to the IDP for licences). These days it often isn't green.
It has a Wikipedia article, see here: https://en.wikipedia.org/wiki/International_Motor_Insurance_Card_System#Green_card_system
Not sure about the "green card" thing, but just a "heads up". From Kotor there is a road that leads away from the bay inland. The views are spectacular. Absolutely beautiful. But, It goes up the side of a steep mountain that has 25 numbered 180 degree switchback turns and at least 75 other sharp turns. Rock wall on one side and nothing on the other. Can't remember the name or number of the road, but it went to the towns of Njegusi and Cetinje. Don't mean to rain on you parade, but don't want you to get into a hairy situation without any knowledge. If anyone in your party is prone to motion sickness or has a problem with heights you might want to avoid this road. You can go to bing.com, use the map feature, and see aerial views of the road from &*$$. You can also check it out on googlemaps.com. It is possible, however, to go to and from Kotor with using that particular road.
On 12 Mar. 2017 there was a thread titled "Greece Montenegro Croatia". One person included a link to the map that shows the road. Another person had a link to a video made along parts of the road.