Santorini or Mykonos

My husband and I are planning taking a tour of mainland Greece for 10 days early September starting in Thessaloniki and ending in Athens. Hoping to add 5 days for islands. In that time which islands are not to miss, how many islands, and is Santorini worth the ferry ride or flight. Also is car rental on the islands preferable to taxi or Uber.

Are you still planning this trip…?