If I were you, I'd head to the Sofitel for dinner and drinks—it’s just across the street and gets you out of the airport for a bit. If you're feeling adventurous, you could take the metro into Athens (about 45 mins each way) and have a quick walk around Syntagma or Monastiraki, but time will be tight.

Another option: take a cab to Rafina for a relaxed seaside dinner, then head back. Otherwise, the A3 lounge or a good book might be your best bet!