Do-Able Itinerary?

Hi. We are traveling to Greece on in mid-September and staying for almost three weeks. I've put together a tentative itinerary but I'm not sure if it is feasible or if I am trying to pack too much in. I would really appreciate some feedback -
Arrive in Athens early morning.
Spend two days and nights in Athens
Take a three day two night tour to Delphi and Meteora
Return to Athens in the evening, stay in Athens and rent a car the next morning.
Drive from Athens to Corinth and into the Peloponeese, sightseeing along the way, ending up in Nafplio
Stay in Nafplio for two nights seeing the sites of the city and surrounding area
Continue south to Monmevnasia, sightseeing on the way
Stay for two nights in Monmenvasia, seeing the sites of the city and surrounding area
Drive back towards Athens; stay overnight in Kinetta, one hour from the Athens airport
Fly to Santorini in the morning
Spend three days in Santorini
Take a ferry to Naxos
Spend three days in Naxos
Take a ferry to Paros and on to Antiparos
Stay three days in Antiparos, visiting Paros as well
Fly from Paros to Athens, overnight in an airport hotel
Fly home the next morning