Would you pick Thasos or Halkidiki for a 7-day trip (gorgeous, less crowded beaches being the only requirement)?
If you're driving, you'll have plenty of choice around Halkidiki, versus probably not allowed to take a rental car on the ferry to Thasos. I won't try to enter the voting for best beaches of the world.
Thanks for the tip Laura, noted! Not sure about the rental cars boarding the ferry, but |'ll check it out.