There's a Germanos shop in Athens airport, on the arrivals level after customs. Number 22 here:
http://www.aia.gr/shopping_center/im...airportMap.pdf
Germanos has a franchise with Cosmote, so you'll only be able to buy a Cosmote card there.
I'd be a bit careful of prepaying for a data plan in advance, especially if you're going to be in the mountains. You may well have signal difficulties, and you don't want to restrict yourself to one particular network if you can help it. I have no idea whether your EUR18/day data plan is roaming, or has tie-ins with particular networks in each country. If it does, you may be out of luck. I very much doubt useful free or pay wifi unless you're staying in upper-end hotels.
So if you really need data, I'd buy a SIM card on each network, and then see what signal you get. Cosmote is usually the best, but it varies.
Are you connecting into an internal flight? How long do you have? To buy cards on each network, you can get a train towards Athens and buy cards from shops there. My first suggestion is get a local 'Proastiakos' train (not a metro) to Neratziotissa. Timetable here, it's about 20 mins each way (and EUR8 ):
http://tickets.trainose.gr/dromologi...s/AER-KIAT.pdf
http://tickets.trainose.gr/dromologi...s/KIAT-AER.pdf
Next to Neratziotissa station is
'The Mall Athens', a huge American-style shopping mall. That has Wind, Vodafone and Cosmote shops, as well as Germanos and others. They're also open a lot longer hours than most neighbourhood phone shops (website only gives me Christmas opening times, but mostly 0900-2100, not sure about Sundays).
Or perhaps locals can recommend places nearer the airport (many of the stations are in the middle of a motorway, so aren't best placed for shops)?
Also, if you have an Android phone there are wireless tethering apps (either in Android Market or native in Android 2.2). It turns your data connection into a wireless hotspot. I've seen the ZTE Blade in WIND shops for EUR169 or EUR179, and unlock codes can be freely generated on the web. Not a cheap solution (particularly since we can get the same phone in the UK for £79), but it's the same price as 10 days of your wifi connection! JoikuSpot is a similar app for Nokia, there may well be others for other platforms.