You can take train or rent a car. I think renting a car is easier as it is more flexible. Delphi is on the way to Meteora. 2 days to visit Delphi and Meteora is not enough. Better take 3 days. First day driving from Athens to Meteora ( stopping by Delphi). First night relax in hotel and get up early the second day to visit Meteora ( it will take a full day.) Staying in Meteora the second night and drive back to Athens the third day.