We are planning on driving from Nafplio to Delphi on July 5th, staying overnight, and then on to Athens. I ahve a few questions:
1. I would rather not book accomodations ahead of time to give us some flexibility in case we decide to stay in Nafplio. Is this ok to do during the summer? My impression of Delphi is that most people see it as part of a daytrip and don't actually stay overnight to it might be ok. We are not looking for luxury accomodations, just something clean and cheap.
2. I noticed when we were at the Colosseum a few summers ago there were a ton of tour guides waiting outside that you could join for a tour, so there was no need to book a tour in advance. Is Delphi similar or do you need to hire a guide before going? Would you suggest going without a guide, and researching the site beforehand, or hiring one? How much should we expect to pay?
3. How long does it take to see the sight and the museum?

Thanks in advance!

Delphi (the town) is tiny and in summer tour buses can take over many of the hotels. I would book in advance and would rather overnight than day trip from Athens.

You don't have to overnight in Delphi, you could overnight in nearby Itea ( on the seaside) or in Arachova that has plenty of Hotels.

What kybourbon mentioned is correct....

Delphi receives many bus tourists from Athens, but also many others that are exploring Mainland Greece.
Do not make any judgement based on this Forum here, as Americans rarely explore Mainland Greece, unlike many European visitors.

Some of the hotels in Delphi have fabulous views so if you do overnight, get a balcony.

Delphi is located in a beautiful setting in the mountains with a view of olive groves that stretch down to the sea. Book a hotel room with that view and a balcony is best.
Take your time driving from Napflion and enjoy your drive and it's sights.

nspotz, I definitely recommend you stay over in Delphi - it' a long drive from Naufplion and if you stay the night before, you can get to the site early in the morning before the day bus trippers. I don't think a tour guide is necessary, but that's my preference. If you get there when they open, you can spend a good 4 hours or so and then have a nice lunch in Arachova on your way back. I felt Delphi to be particularly magical! Many of the hotels cater to bus tours, so you might find a place in Arachova that will be more quiet. I would think you should reserve ahead in the summer.

clausar, I was disappointed to note your comment that Americans lack helpful advice.

Thank you! It seems the best thing to do is book in advance, so we will do that!

I have another randome question: I thought I read somewhere that alot of stores and attractions are closed on Sundays in Athens. Our last day in Athens is a Sunday and I would hate to miss some things because of that. Is this true or did I mis-read? Thanks!

Just a suggestion.... have you considered including Meteora in your itinerary?

In the historic centre of Athens, in the Plaka and Monastiraki shops will not be closed, in Monastiraki there's a flea market every Sunday and lots of people visiting, which is very interesting.

Going up the Ermou Street towards Syntagma Square, shops will be closed...
The New Acropolis Museum, the Acropolis and the national Archaeological Museum will all be open on Sunday.
For the opening hours you can check the official website of the Ministry of Culture : http://odysseus.culture.gr/index_en.html

parosblue
I am really sorry if i a disappointed you , i didn't mean that Americans lack helpful advice, all i said is that most people on this Forum have limited or no experience from Mainland Greece with the exception of a few very well known destinations.
When i read post of other countries i always like the way people explore countries like Italy or Turkey, driving even to very remote places.
Unfortunately the interest in Greece is limited to 3-4 islands ( Santorini, Mykonos, Naxos and Paros and sometimes Crete) and when it comes to the Mainland : Nafplio and the Argolis plus Delphi...

I am sure you know that Greece is much more than that, and i wish that people would start to explore it more and more.