Itinerary Help

From Delphi to Olympia, visiting Olympia, and then driving on to Nafplio is likely too much for one day. A lot of miles, not all on the best roads. I'd suggest arriving in Nafplio Monday and exploring the town and hilltop fortress. Epidavros and Mycenae would take all of Monday. You could easily spend another enjoyable day, Tuesday, in Nafplio. There's also an all-day boat ride from nearby Tolo to Hydra, Poros, and Spetses (Pegasus Cruises), but Santorini may have been enough island for you. Others on this board recommend Nemea, I haven't been. I would recommend Corinth on your way to Athens -- the ancient ruins, perhaps lunch overlooking the ruins, and a brief stop to see the canal. You can easily fill the three days you have for Athens, and if you get there sooner, like Thursday, so much the better. Hope that helps.

You have a pretty reasonable plan. I can attest that the drive from Delphi to Olympia is a very long day. You would definitely have to stay the night at or near Olympia. You might then consider slipping down the coast and visit one of the villages of Pylos, Mythoni and Koroni. The all have wonderful Venetian era Kastros. Spend on of your free days in this area then drive to Nafplio. Add that last free day to Nafplio. Nafplio itself requires a couple of days with its old town and Venetian kastros. Epidavros and Mycenae are a good use of that other day.
Olympia Delphi Meteora https://www.flickr.com/photos/stanbr54/sets/72157645469717811/
Peloponnese Battle Castles https://www.flickr.com/photos/stanbr54/sets/72157645468134284/

I fully agree that a stop in Corinth on the way back to Athens is a good idea. Ancient Nemia is just a 5 minute drive off the the National highway. It had sister games to Olympia and still has its stadium a partially restored temple and a very good museum. The site can be done in an hour or so. Then head over to Ancient Corinth It is another compact site but a very good one. Again about an hour and you can be back on the road for a visit to the Canal and then back to Athens.
Nafplio and Peloponnese http://www.flickr.com/photos/stanbr54/sets/72157632094108982/

Having said all that if you are absolutely set on Olympia then I would do it exactly as I outlined. However if you are wavering at all about that long drive to Olympia and are willing to give it up then this is what I would do.

Depart Delphi and cross the gulf of Corinth at Patras and head for Nafplio. You can do Corinth this day and still be in Nafplio in the afternoon. Definitely add those two extra days to Nafplio. You might consider visiting ancient Tiyrns and perhaps Argos and perhaps the long day trip to Mystras. Since it is going to be off season there is a chance that Mystras may close at 3 o'clock so you will need an early departure for the round trip. Here is an idea, you could use one of your extra days for the Mystras trip. Visit Mystras( the site takes about 4 hours) then continue south and east to Monemvassia. Spend the night there. The hotels have period rooms scattered around the walled town. It is an amazing experience to stay there. You can drive back to Athens from Monemvassia for your three days in Athens
All of these sites are in the Nafplio and the Peloponnese album.
Athens http://www.flickr.com/photos/stanbr54/sets/72157632121475515/

You'll love Oia, it's wonderful, and I'm happy to see that you'll be spending enough time there to relax and enjoy the timelessness of Santorini.

You won't have any trouble finding accomodations in October anywhere in Greece so just go with the flow. Don't make plans for Olympia, wait until you're there to see how you feel about the drive and if it's in your best interest to go there. If you decide on Nafplio instead you won't be disappointed.