Crystal Cruises, a luxury cruise line, is offering a number of cruise possibilities for 2010, including trips beginning or ending in Athens. These options include business or economy class air and $500 -$1000 in on-board spending credits, making the about $3800 per-person fee start to sound surprisingly affordable.
I like the looks of the Seaways of Antiquity option, which starts in Rome, makes a couple of Italian port calls, and then touches at Crete and Mykonos, plus the ancient Greek city of Ephesus in Turkey, before ultimately disembarking in Athens. The reverse route, ending in Rome, is also available as the "Parthenon to Pantheon" cruise.
And these cruises are not stuck in the off-season either, when weather can make the best of cruises a bit awkward. The cheapest May dates should assure good weather and position you perfectly to enjoy a nearly crowd-free, low-cost stay in Greece either before or after the cruise.