Came here to say the same thing, it's either Laurel or Oleander leaves that Pythia (the high priestess) either chewed or inhaled (their fumes). At least that's what I remember from high school and what most Greeks know. It's debated that the whole Oracle of Delphi thing could be a very well organized scam, but that's another discussion.

It's a place worth the visit: https://www.visitgreece.gr/mainland/central-greece/delphi/