DYI daytrip to Delphi?

It's usually not possible to buy Greek bus tickets in advance, or even to confirm the schedule in any reliable manner. Plan to leave early and buy tickets at Athens bus station B.

thanks much for the tips...however, I am trying to stay away from the bus tour, and another new friend just suggested going to Delphi on a mid-day bus ( get the return ticket), seeing the museums in the late afternoon/evening when they are almost empty, spend the night in Delphi, see the ruins early in the morning before the crowds arrive, then have lunch and bus back to Athens early afternoon.. That would then be my final night in Athens before flying home. sounds like a plan. :)

Seeing the museum late one day and being at the gates at 8:30 when they open is what I usually do. It avoids the crowds at the archeological sites.

Your plan is good. You buy the bus ticket that day before you go (and get the return). You can also buy it a day in advance if you were going at a popular time, but a mid-day bus should be fine.