As a result of the very poor getyourguide site (and you had one job), I picked what they called the "most popular" day trip which was indeed to Delphi only. Once on the trip, driving through Arachova and realizing how beautiful it was and why it was on the other tour that I didn't pick because I had no idea, now I had deep regret. Thankfully I was able to work something out to rescue to the day, do both Delphi and Arachova (With 2H in Arachova), and still catch the bus back to Athens. Let's start with Delphi, though. It was another one of those pretty impressive places back in the day, of which there is only so much left today, so you have to read the descriptions and use your imagination along with the audioguide:
found a beautiful and probably poisonous spider on the way ;)
the top arena was not well maintained and probably hadn't been for a while