As someone with some deformities leading to way too many foot problems, shoes are foremost on my mind too for my May Greece trip. I know there'll be lots of hiking up and down and Rick has warned of the slippery cobblestones everywhere. I'm therefore considering taking hiking poles as well as good shoes.
I have wide toes, so many Keens, with their wide toe box, have been good for me. For all purpose use, including limited walking and going into the sea, I'll be taking my Keens Newport waterproof closed toe sandals.
But for hours of walking, the Newports won't be comfortable enough for me. I'll need full fledged walking shoes with socks for cushioning and support. After going through many brands over the decades, I've recently discovered Altra to offer D widths and be very comfortable, and will be taking a pair to Greece. A natural shape with wide toe box, very light and comfortable, and the mesh uppers allows cooling air flow. I'll be taking a pair of pink Superfeet OTC orthotics to insert for more support, as needed (they work better than my 3 custom made orthotics!).
I've also had good luck with Asics Gel Nimbus in wide and will have to take that too. REI and Sports Basement, a Bay Area chain with an online store, consistently have well trained and helpful shoe staff that have helped me find good shoes. In addition, both will allow you (even encourage you) to actually try out your new shoes outside and will still accept them for full returns within a certain time frame (1 yr. for REI)! So useful!
As I find socks make a big difference in comfort, I also vote for Balega. My feet tend to sweat, so I need something breathable and light but with cushioning like these. I have many Smart wool socks, but they wear out quickly and would be too warm for me during hot weather.
Good luck Julie, I hope you float through Greece on comfortable shoes!