If you go landside, there's a nice Sofitel hotel right across the roadway in front of the terminal. It has a restaurant and IIRC a bar or coffee place with some outdoor tables as well as inside air conditioned seating.

You can check for PP lounges on the PP website and lounges for AmEx Plat cardholders on the AmEx website. There's also a forum here on FT devoted to airport lounges, although IIRC the AmEx Centurion lounges are discussed in the AmEx forum. Check also for information in the Greece forum. Note the distinction between the domestic/Schengen and nonSchengen areas of ATH when investigating lounge locations.