Are you thinking of visiting Greece, irrespective of the economic crisis? I was there when the potential for Greek default was announced. Like I told the Seattle Times, I left Athens the very next day and it was a challenge—nobody was accepting credit cards. I traded my iPad with a cabdriver for a lift to the airport because I just didn’t want to be in the country any longer.
There are still many tourists heading to Greece, especially if there’s a special event happening, if the trip was planned long ago or if it’s expensive to change/cancel it. If you do risk it, make sure you take plenty of Euros in cash to avoid non-working ATMs and businesses that can’t/won’t accept cards.
If possible, pre-pay as many items as you can like hotels, events, outings, and adventures. There are still many options for booking online, and that can save you a big headache. Make sure you trade your USD for Euros at an American airport before jetting off. Most importantly, remember that a shaky economic means uncertainty in plans, so be flexible.