The obvious precaution is to take plenty of cash and split it between you to reduce the risk of loss or theft. If you've any choice in the matter, pick a hotel with in room safes. Maybe check how much cover your travel insurance provides. To get the best rate - and the euro has crept above 1.40 to the £ this week - consider ordering in advance for collection at the airport.
Will plastic be accepted? There is a possibility that at some point ATMs may not function, though the immediate danger appeared to have receded and one credit card company notes that it would be possible for banks to continue allowing the use of foreign debit cards. Credit cards should be accepted - but as payments go into the bank accounts of traders including restaurant, bar and hotel owners they may be much keener to be paid in hard cash.
Should you cancel your holiday? No. Provided you take note of these precautions there's unlikely to be serious disruption. If the worst come to the worst there won't be sudden switch back from the euro to the drachma. Greek life must go on. Besides, they need your money more than ever.