Okay, let's start with the international driver's license. There is no such thing as THE international driver's license. These quasi-translations of your national driver's license are issued on the basis of intergovernmental agreements. And there are several of them. The Vienna, Geneva, and Paris Conventions regulate the recognition and issuance of international driver's licenses. In Germany, for example, we get international driver's licenses under the Vienna and Paris Conventions. But for example the US and Canada are signatories to the Geneva Convention, which Germany has not signed. The good news for travelers from Germany is that the US authorities generally accept an international driver's license in accordance with the Vienna Convention in conjunction with a valid EU driver's license as a gesture of goodwill. I assume that the same applies vice versa. I have read that in some EU countries also accept the national US one for short term stays but I think you will need the IDP to rent the car. Better go prepared. You have to check.
Otherwise, I can't think of any permits you would need to apply for at the moment. But another important thing to consider are the low emissions zones in various cities or regions of Europe. Each country has its own regulations in this regard. In France and Germany, for example, a sticker on the windshield must prove that the vehicle complies with the regulations for entering the respective zone. Rental cars usually have these stickers. It's best to check with the rental company to see if they have them for the countries you want to visit or if you need to take care of it yourself. In France, you can apply for the sticker online and have it sent to you. In Germany, you can get it by presenting your vehicle documents at garages or technical inspection stations.
Well, and there are different toll systems. In France, you usually pay at the toll booth on the highway, while in Switzerland or Austria, you have to buy a vignette in advance (online or at sales outlets before the respective border, for example).
Travel health insurance is always a good idea when going in a trip. Speaking of health, it seems important to note that in Europe, for example in southern parts of Germany, there are so called risk areas because of ticks. Ticks in Europe can potentially transmit Lyme disease or TBE. Therefore, it may be worth to consider protective measures. This is a personal matter that everyone should address for themselves.
August/September is still peak travel season, so campsites can indeed get quite full. However, there are plenty of places, and if you don't need a specific one, I think you'll probably find a good spot. What might make sense is to call ahead, perhaps the day before or in the morning, if you want a specific spot, to ask or make a reservation.
I am not up to date with ETIAS. The launch of the system has been postponed several times since 2020 and is currently planned for the end of 2026, as far as I know. I would wait and see for now.
I hope that also other members of
Bill of Sale from Europe will get in touch and give you information and tips. Enjoy planning!