It is not impossible, in your time frame, to fly straight to Mykonos or Santorini (not nonstop but one change). They might not have sea bathing but they also might not be as overrun as in the summer, depending on your timing relative to Greek Easter. Two or three nights - get over jet lag, if on Mykonos take a day trip to Delos for the ruins, if on Santorini hike and enjoy the views.

Flight to Athens, pick up a car at the airport, off to Nafplio and the many attractions within easy driving distance.

End with as many days in Athens as seem good to you.