It depends on what you want. If you want to have nightlife, you chose Santorini or Mykonos this time of year.
If you want something very near Athens since you are having a car, I suggest Syros or Egina. These are very near Athens.
An Island with LOVELY BEACHES is Skopelos and Skiathos or Mitilini.
Another nice destination since you are having your car along, is to come to Evia. If you look it up on the map you will see that it is actually a big island and you can drive there because it is connected by a bridge.
You can drive from Athens and come to Chalkida in just an hour if it is not a busy day. Otherwise it is just 1.30 minutes. (80 kms) Both north Evia and south are beautiful. You can search in my blog and find entries of Evia with hundreds of photos. There are places even for camping, like Rodies. This coming Tuesday I am driving up to north Evia, and I am going to have hundreds of photos from different places and I am actually going to visit a new spot this time. You will have to look up for the photos in my blog this coming week.
I have entries in the first and 2nd blog which I am sure you will find interesting.Since you have already visited many Islands as you said, and since you are having a car with you, why don't you get to know the mainland. Find Livadia up in the map of Greece, which is about 150 kms from Athens, and then go on to Arachova. A nice resort both beautiful in summer but in winter as well.
On the way there you can visit Delphi.(Hundreds of photos in my blog)
Also if you drive about 400 kms from Athens you can visit Meteora.
But there are so many more beautiful places to visit. If you need more information get in touch again and let me know.http://www.travelpod.com/members/greekcypriot
My LATEST BLOGS: Marmari, Vasa, and CYPRUS, the Unique Destination for Business and Holidays!!!
The World is a book, and those who do not travel read only a page. ~St. Augustine