Currentl plan is to arrive March 21 spend a few days in Athens then pickup rental outside of Athens for a tour of Peloponnese. Nafplion, Kardamyli, Olympia each for a few days. Then to head North up thru Patras, to Ioannina, Meteora, Mt. Olympos and possibly Thessaloniki. Then back to Athens for April 9 departure. It is quite a lot and I am not too familiar with Patras to Mt. Olympos travel and looking for some advise.
YOur project is to get familiar with the territory ... and you cannot expect to accomplish that with reading a few travel forum posts. Go get yourself a couple of Guide books. R. Steves book will help you with Athens and the Peloponnese. The other Guide book that is thorough on ALL sections of greece is ROUGH GUIDE. You don't need to have the very latest edition, because y ou won't depend on a guide for the latest hotel and restaurant prices. Go look at Amazon, new & used.... $10 - 18 ... and get reading. Then go to Michelin Route-planner, which will show you the BEST driving routes (including drive time, petrol costs, tolls). When you have a bit more know-how under your belt, you can ask us specific questions, not need a complete Mainland Briefing. And research is FUN! Best wishes, look forward to your specifics.
I am more familiar with the northern areas you name. The trip will be beautiful, but just keep in mind that the driving will be tiring; you will not be in an easy cruise control situation on big expressway. You will sometimes be on two lane roads with the center serving as a passing lane. Driving is aggressive in Greece, 10 km can take 30 mins. if there are switchbacks or climbs, etc. You might think you can drive 6-8 hours per day, but just keep in mind that it will likely feel much longer because of these added issues. There is a major newer highway from Ioannina going eastward toward Thessaloniki; lots of spectacular views with steep gorges, valleys, tunnels that are many km long. Thessaloniki is a great city with wonderful museums, ruins, and some of the best cuisine in Greece. Enjoy!