As part of our longer trip, we are planning to drive up to Delphi in the evening, stay the night, tour Delphi for a day, then stay for another night before heading back to Athens. Is that too much time? Will we run out of things to do? Should we stay one overnight, tour for most of a day, and then drive back to Athens the same day?
Delphi as a town has zero charm (although some awesome views). You can easily see the archeological sites and museum in a half day or just over. But there area does have some other things to see, such as the Byzantine monastery of Housias Loukas, the little seaside town of Galixidi and some scenic mountain drives. So if you stay two nights, you can see the sites at a more leisurely pace and fill your time with other things. The other option is to spend a couple nights in Galixidi, a much more charming town. It's about a 30 minutes drive from Delphi.
We stayed two nights, on a more leisurely trip than yours (as described in your other post), but I agree with Douglas that just one night would be enough. Lots of people do it as a day trip from Athens, which I'd call way too rushed. We liked the town of Delphi, but I don't know about Douglas' other suggestion (Galixi?).
We also stayed 2 nights. There is a good solid day of sites to see. One night is probably enough.
It depends on when you get to Delphi... AND what time of year (museum may close early in the winter). If it's late spring thru fall, then it's nice to arrive late in day, get a small hotel w. view down the mountain (I like hotel Pan). Then, walk to museum if it's open until 7 or 8, there is NO ONE there! IF there is time, you may also go further downhill and enter the "tholos" site (the circular columned ruin seen on all the posters)... that area is open 24/7. You can also find the famus Springs down there. Back to your hotel, walk to a nice terrace cafe overlooking the downhill enjoy sunset at dinner. In morning at 8 or 8:15, enter the ruins -- almost no one there, you will have a solid 2 hours before any bus tour groups arrive.... BTW, if you make the steeeep climb to the stadium in hopes of exploring u will be disappointed, because it's been closed several seasons due to rockfall. In my opinion 2 hours in peaceful ruins, is ample ... Thus, you could do it in 24 - 28 hours and not feel rushed, with attentio to timing (leave ATH noon or 1, 3.5 hours drive, next day leave site at noon, back in athens before 3).