I second the Sofitel idea. I'm not sure if you have access to the NS lounge, but even with that I'd exit to the Sofitel and then go back and head straight to the A3 NS lounge after clearing security.

It's a pity that there are no other hotels at the airport itself. The Sofitel has no competition and charges usurious rates (usually Accor places are more competitively priced than Marriott or Hilton equivalents of a similar standard). How can the biggest airport in Greece have only one hotel?

I need to do an overnight in ATH for a trip to IST on 31 MAR but I've booked the nearby HI instead on points.