Some etiquette in Greece tips regarding clothing: Do show a modicum of decency and use your common sense. Carry a shawl around with you that can be used as a cover for legs and shoulders if needs be. Don't show your bare legs or shoulders in a church, especially not a monastery, and men: please keep that singlet on.
What do men wear in Greece in summer?
Men tend to wear tight trunk style shorts and 'speedo' briefs but plenty of visitors happily wear boardshorts. Pack at least 2 pair in summer as you will wear them a lot! A sunvest or 'rashie' is also good idea, especially on the first few days to avoid serious sunburn.