Careful changing money in Kos -- especially at the Western Union on the corner near the ruins in the center of town. Everywhere else I went in Greece, the people were great -- they charged official prices with price tags and receipts, and were very open about making sure that tourists were happy with their cultural experience in their beautiful country.
This one shop in Kos is unfortunately the exception to the rule. Granted, most of this is my fault for being too trusting. But, I had already changed money three times in Turkey where they publish the rates, gave me a receipt and there were no problems. In Kos, at the Western Union, the older lady there simply took advantage of my trust. She did not quote the rate up front and didn't give me a receipt. it wasn't until I returned to my room that night that I realized how much I had been ripped off. When I did the math, instead of losing 3-5% as you normally expect during a transaction, she "charged" like 35%. Again, my fault for trusting her during a cash transaction, but for a country that is incredibly dependant on tourists, it's a shame to see one "bad apple" who could have soured my whole experience on the country.
So, remember the three basic rules that I neglected -- confirm the rate before you start the transaction; then ask for a calculation of the specific amount you want to change; finally, make sure the calculation is official and that a receipt is produced. Do these three basic things, and you won't be taken advantage of by anyone, regardless of their motivations.
And, if you every happen to be in Kos, don't use the Western Union there and give that lady the stink eye for me as you walk by.