We spent 10 days in Greece in June and loved it BUT we based our trip around Rick Steves' Athens and The Peloponnese Guidebook and felt the itinerary was too driving-based. Unfortunately we spent more time in the car, traveling place to place, than actually enjoying it. My husband and I both lived in Europe and have travelled extensively. Because we had our 3 year old and 1 year old children, we didn't even incorporate Olympia or Delphi. The Greek people were among the most welcoming, kind, kid-friendly people we've ever met.
Here was our itinerary:
Santorini - 2 days (very expensive but our favorite place on the trip was Imerovigli town/Oia was WAY too touristy. Rick doesn't even suggest going to islands but the geography of the Cyclades are not to be missed.)
Flight to Athens: Drove directly to Costa Navarino took 3.5 hours (spent 2 nights)
Kardimyli - 1 night (Drove 3 hours) Cute town but not worth more than a drive through. We planned to stay in Rick's recommended Kalamitsi hotel but ended up staying only 1 night because the manager was not very nice, rooms REALLY basic/hard beds, and they charged us extra for breakfast when they told us it was included when we made our booking.
Mani peninsula - 3 nights. LONG drive, extremely barren, rubble buildings. Coastline is beautiful but few easy access points. Would have stayed in Limeni if we had it to do over again. Gerolimenas' Kyrimai hotel is lovely but not good for kids. Rick recommends doing the Mani in a day but it is a LOT of driving (6 hours?)
Nafplio - 2 nights (Drive 4 hours) from Mani. This was our favorite place aside from Santorini. Small enough to walk everywhere, charming old center. Epidavros is an easy 30 minutes away
Athens (1 night): We stayed in airport hotel because of our early morning flight but would have spent more time in Athens if we were to do it again.
In hindsight, our itinerary would have been 3 nights in Santorini, another 2 on a smaller island, 2 nights in Athens, 2 nights in Nafplio, and incorporated Delphi. We would have skipped Peloponnese altogether or only gone to Limeni. If you have lots of time to drive, then perhaps its worth it. We must have spent 14+ hours in the car and we didn't even include Delphi, Olympia, or Monemvasia.
*Rick's recommended Swift car company was so helpful (picked us up airport) but make sure to ask for a newer car because the one they gave us was so old the air conditioning died in the hot (!) and REMOTE Mani peninsula and we were afraid it wouldn't make it to Athens!