Hello everyone,
Soon I'll have a long layover with A3 of 6:10 hours in Athens, in the evening arriving at around 17:30 and departing at around 23:50. The arriving flight will be intra Schengen, however the departing flight will be international and therefore I will have to clear passport control (that being said i am an EU citizen so it shouldn't take long). Do any of you have any suggestion on how to spend those hours inside (or outside) the airport?

Unfortunately, nothing comes to mind given the combination of time of the year and time of the day (if "soon" means within the next 10 days or so). The sun sets around 6 p.m., preventing you from visiting archaeological sites in Athens or a nearby beach. Moreover, the evening rush hour would make you nervous unless you take a train/metro. By the way, to get to the centre of Athens you'll need about 45-60 minutes, one way, which leaves about a maximum of 1-2 hours to walk around the centre of the city. The only possibility (although too early in the day for that) I can think of is to take a cab to Rafina, have an early seaside dinner at a taverna (weather allowing) and then back. Bottom line: I'd suggest to "arm" yourself with some good reading, one or two crossword puzzles and... patience!

Walk the 50 yards over to the Sofitel and have a drink and dinner.... gets you out of the airport for a couple hours.

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.

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!