We ask our hotel for a recommendation for AUTHENTIC Greek food, and were pointed to Paparouna. It took us a little while to find it, but it certainly was worth the search - in fact, it is probably the best meal I've ever eaten in Greece. We ordered multiple appetizers and a Greek salad, and all were excellent. The wait staff was very attentive, and gave us excellent recommendations on what to order. At the end of the meal, they all brought us complimentary desserts and an aperitif.
If you have only one night and one restaurant to pick while in Greece, this is the one. Don't miss it!