I second George's advice.
A day trip would be far to short to come to Crete and you might get the wrong impression of the island, especially if you stay in the Little Monstrosity they call Iraklio, my home town. (the Large Monstrosity is Athens btw). Crete is not as large as Sicily but it is still the biggest by far island of Greece. With its varied topography it is almost like a tiny continent in itself. Check out a post I made in the geographical sub-forum regarding the island and my open invitation to all APUGers.
A plane will get you to Crete from Athens in 35 minutes but you will have to go through the airport which is a waste of time in itself. The boat is even more time-wastefull as the fastest ones take atleast eight hours as George mentioned, let alone taking the older ones that need atleast half a day.
It is though very romantic and nice and a photographer could appreciate such a trip, especially if you come upon a circuimstance simular to the one the photographer in the movie The Brides gets himself into.
I suggest explore Athens, the old city around Acropolis that is.
The rest of the city is too chaotic and too visually uninteresting apart from a few certain landmarks.
Just take care on the streets, subway, buses and taxis. Greeks, especially Athenians are very self-involved, extremely rude and have no basic street respect knowledge; they will push you and elbow you and step on you to get where they are going.
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