All U.S., Canadian, Australian, and New Zealand citizens, even infants, need only a valid passport to enter Greece for stays of up to 90 days. If you leave after 90 days and don't have a visa extension, you will be fined anywhere from EUR 130 to EUR 590 (depending on how long you overstay) by Greek airport officials, who are not flexible on this issue. Worse, you must provide hartosima (revenue stamps) for the documents, which you don't want to have to run around and find as your flight is boarding. U.K. citizens, like all EU nationals, need a passport but can stay indefinitely.