I highly recommend staying overnight in Delphi so you can experience the ancient ruins without all of the tourist buses. Husband and I drove up and got to Delphi around 1pm - planning to spend the night as it is about 3+ hour drive from Athens. We headed to the museum/ancient site around 2pm and were dismayed to find that the site was jam packed - in September! So, since we had not had lunch, we decided to have a very early dinner and returned around 5:30 (site closes at 8pm). Amazingly there was NO ONE around. I am not exaggerating when I say that we did not see more than 10-15 people within the ENTIRE site - which is half of a mountain top - compared to 100's of "bumper to bumper" tourists at 2pm. (tour buses leave by late afternoon to go back to Athens/next tour stop). By silent agreement, we all spaced ourselves out so that it was possible to experience the site as if there was no one else there. WOW!!!!
Good report, Iloveparis .... and that is exactly why I recommend people who are D-I-Y visitors to Delphi via bus, to arrive late afernoon... the crowds are GONE! You can go into the museum (VERY good signage) and it's EMPTY. we then walked down to the Tholos tomb below the regular ruins site... then went & had a sunset dinner. Next AM we were at the ruins gate at 8 AM and had plent y of time to explore the ruins before the busses arrived around 10:45 ... we got a midday bus back to athens. You could also do as iloveparis did, & visit the ruins in early evening & t he museum in the AM.