A few things you should know:

I'd like to share a few tips with you that have served me well. The following information is a mixture of my experiences and what I've read from other sites. A good place to begin your research is www.lonelyplanet.com/greece .

You should exchange some money either before your flight to Greece or at the airport or you won't be able to get a cab or a drink and so on. Lonely Planet recommends that the majority of your money should be exchanged at a bank where you'll get a better rate and pay less commission. For safety purposes exchange only what you'll need for the next few days, about $300.

Tip: Credit cards are widely accepted in major towns but it doesn't hurt to ask rather than assume that this is true in your case.

US Embassy in Athens: Though Greece is not in the middle of a war it is still a foreign country and you should know where the embassy is and how to contact fellow Americans if necessary. You can also check out the embassy's site to learn about U.S. and Greek relations:

U.S. Embassy Athens
91 Vasilisis Sophias Avenue
10160 Athens, Greece
Phone (Main Switchboard/Info): 30-210-721-2951
E-mail: AthensAmEmb@state.gov Web site: http://athens.usembassy.gov/

Tip: Always carry photocopies of important information, such as a copy of a passport, in case it is lost or stolen. It won't get you back on a plane but since you still have something to show from your original it might make getting a replacement easier and faster.

Money: The monetary unit of Greece is the Euro. Euros reflect the country of origin's culture. Since Greece has a lot of toursim it is reflected in the variety of the Euros. For more information on Euros take a look at:
http://en.wikipedia.org/wiki/Greek_euro_coins
www.fleur-de-coin.com/eurocoins/coins.asp

Tip: Check out your Euros and see if you can figure out where they came from. Keep some as souvenirs or for your scrap book.

Markets: Stores do not operate the same hours as in America and they are not open on Sundays except for the kiosks on the corners. Depending on where you are in Greece businesses close at 2 and may not reopen again in the evening. Don't be afraid to haggle for things that seem overinflated price wise. Bargaining is not an unusual practice in Greece with street vendors.

Tip: Plan ahead and buy bottles of water or fresh fruit while stores are open.

I have had several experiences where I was not given all of my change in Greece. Most people outside of Greece expect to be given their exact amount of change just as buisnesses expect to receive every cent they are owed. In Greece businesses can't be bothered with 1 or 2 cent coins nor will they insist you give it to them if you are short.

Life in general: Greeks typically nap after lunch around 2 p.m. for 2-3 hours. The only people on the beach at that time are foreigners. It is the hottest part of the day and the most dangerous in terms of sun and dehydration.

Tip: It is considered rude to make noise during this time whether you are at a hotel or in a village. In case your kids can not sleep try some good quiet activities like drawing, reading, journal writing, planning out the next day's activites or watching a dvd with headphones on.

Pharmacies and health: All of the pharmacies take turns being open on an emergency basis. Your hotel should know which pharmacy can help you the moment you need something. You should have a list of your medications and dosages. Pharmacists can dispense many medications without a doctor's note. Be prepared, pharmacists and doctors may not recognize the names of your medications as they may be marketed under different names abroad. If you can, pack the pamphlet that comes with a medication to be on the safe side.

Tip: Buy a thermometer that measures Celsius and Fahrenheit before you leave and take it with you.

Medical staff in Greece typically do not understand Fahrenheit and you may not understand Celsius. Here are a range of temperatures from normal to fevered from http://fahrenheittocelsius.com/:

°C to °F
36=96.8
37=98.6
38=100.4
39=102.2
40=104

Tip: In addition to the thermometer you may want to use the pain/fever reliever most common in Greece. Ask the pharmacy for directions and clear up any confusion before you leave. If you need further assistance at a later time you can explain to medical personnel what you have been using so you are not overdosed.

Dining out: You should consider dining where locals eat and where prices are clearly stated. Food is never served "rare" but is usually well done. For additional information on tipping etc. take a look at: http://gogreece.about.com/library/weekly/aa031398.html