I have a tentative itinerary sketched out and would appreciate thoughts, suggestions, and feedback.
Athens: 6 nights (we arrive February 17, late afternoon and I'm planning 5 full days)
Crete for 7 days (fly to either Chania or Heraklion, depends on flight times and cost). We will stay in one place 3 or 4 days, hire a car and then explore and end in the other town
Athens (fly back, pick up a car, possibly overnight if that's how the best return flight works out)
Peloponnese road trip (10 -14 days, nothing planned, will probably book lodging once we're in Greece, possibly sooner)
Thessaloniki (3 days and take 2 days each way to drive as it's less expensive than flying and we like road trips although we're also considering taking the train both ways)
Athens possibly one night depending on when our flight to Croatia is (we haven't planned that yet). This would be around March 25
That's about 5 weeks, give or take a day.
A couple of months ago we got a great deal on flights: fly into Athens from the east coast of the US and out of Zagreb to Boston with a layover in Istanbul. At the time we thought 5 weeks in Greece, 2 in Croatia driving from Dubrovnik to Zagreb. We are now open to extending Greece and shortening Croatia. We could just do a few days in Zagreb which would give us about 10 more days in Greece. Our flight from Zagreb leaves April 9.